The book "There Is No Memetics Division", if they have a bookshelf.

Minimal spoiler: It is about induced memory loss.

I saw someone do this to a neighbour and i thought it was really funny and not harmful. If you can then take the photos they have already put up inside their homes and photoshop them. Make their forehead bigger than what it already is, make someone smaller of taller than they are and stuff like that. If possible you can insert yourself in one of the photos. Then take the photoshopped photo and put it in front of their own photos inside their frame. You could also write a message behind the manipulated photo.

A while ago I realised that blackboard chalk sticks are about the same size as cigarettes.

Next time I visited my friend who smokes, I waited til she was out getting us dinner, and I replaced all the cigs in her pack with chalk sticks, then hid the cigs somewhere else. Later that night after I got home I got a text from her "Where the fuck are my cigs?!"

Very funny and mostly harmless

Oh. Oh god.

Okay, so bear with me.

Many years ago, some friends and I worked at the University Museum of Archeology and Anthropology at University of Pennsylvania. At the time of this story, the museum was undergoing a bunch of renovations. The renovation dust would intermittently get kicked up by the ventilation system or would fall in a clump or whatever, and the movement would set of the security alarms. After a couple months of this, Security became somewhat lax in responding to alarms, because every night there were a number of false alarms.

So, one early morning, a student is walking to the university and spots something weird sitting in the middle of the pedestrian walkway on the South Street bridge. As they get closer, it starts to look familiar. They get up close and recognize it as the solid silver stand that usually supports in the Rotundra of the Museum. This is the first indication that anyone has that the museum was burgled the night before.

The police are called, the stand goes through evidence collection, everyone traipses over to the Museum, Security (and the museum administration) is shocked. Everyone starts looking around for whatever else might have been taken. Eventually we conclude that only three items are missing: the Dowager Empress' crystal ball, the solid silver stand the ball usually rests on, and a 2500 year old bronze statue of the Egyptian god Osiris. [The ball is gorgeous: it's like the third largest crystal ball in the world, it's absolutely flawless, and John Wanamaker bought it for like $50,000 back in 1920.]

All three items were taken from either the Rotundra itself or just nearby. This is somewhat confusing, as the Rotundra is all the way toward the back of the Museum, and up a couple staircases. Why wouldn't the thieves grab stuff from a more accessible area instead of crossing almost the entire Museum? Also, the objects are heavy or difficult to carry - why wouldn't they take something smaller, more easily portable, and more valuable - for example, the Tang dynasty horses that are also in the Rotundra and much more valuable?

Even more confusing is why the thieves decided to ditch the stand for the crystal ball. Presumably they were having problems carrying all three items and decided to leave one behind. Did they drop the four-foot tall 60-pound statue of Osiris? No. Did they leave the 50-pound crystal ball which is very round and can be hard to hold safely? No. They decided to leave behind the 20-pound stand, which has lots of easy finger-holds, is made of solid silver, and is easily meltable into easily-sold unidentifiable metal. It's all just ... very weird.

Anyway, the police show up. Because of some University association with the state that I can no longer remember, the FBI shows up. There's lots of chaos. And ... nothing happens. The FBI takes the stand into custody for forensic examination, but they can't find any clues. They keep the stand in custody for a couple years in case "something else turns up" but the case goes cold.

Eventually they return the stand to the Museum. We clean it carefully and, heartbroken, put it into storage. And for a couple more years, nothing happens. [continued]

Coins. It's not a weird item but if you hide hundreds of 1 cent coins around the whole place at some point they will wonder where all those coins are coming from.

Steam up the windows/mirrors and write something. It's only visible when the room steams up again.

"Help," "I'm sorry," "I miss you," etc.

I also used to buy wall decals. You can buy Mario themed ones or whatever and leave them inside cabinets, behind doors, etc. The right kind comes off without leaving any residue.

We use to leave pictures of Nicolas Cage everywhere. Print them out like 30 to a sheet so they're small and discreet but plentiful. I think the most I his before getting caught was 80. Only a few left in my pocket before one was discovered. He found them for months afterwards. This was about a decade ago.

This is one I was planning to do for a friend but he moved away and I never got a chance.

Toast his bread. Take two slices of bread out of his bread bag, toast them, then put them back in the middle of the loaf.

You can buy a sheet of "for rectal use only" stickers pretty cheap and you can put those all over

1000 tiny plastic babies

Mini rubber ducks

My go to is always sneak in a framed photo of Peggy Bundy in amongst their own photos. She is Americas mother after all!

19 sticky notes, label them 1-10 and 12-20. Hide them in random places with 1 modestly easily found on a regular day.

I'm a big fan of the cress keyboard.

If they don't have a cat then leave a litterbox, cat toys, and cat food out as if a cat has been living there. If they don't have a dog then leave a leash, dog food and dog bed. Repeat for other animals you get the idea

I love all of these suggestions, they're delightfully unhinged.

As for a suggestion, maybe get a bulk pack if "elf on a shelf" elves, and hide them in rarely checked areas; under the bed, on top of cupboards, right at the back, taped to the bottom of the dining table, under the couch cushions, and the like. Give them cryptic notes to hold if you can think of enough.

I used to work for the federal government and we all HATED a certain department manager. It was a huge facility with hundreds of employees and had an institutional sized break room with several glass door commercial refrigerators, like you see at convenience stores. Employees had to have their names on all refrigerated items or they would be thrown out. Somehow I wound up with a bottle of chocolate flavored edible lube, so I put the department manager's name on it and stuck it amongst the bottles of condiments. The department manager was too good to eat in the break room with the peons, so she never saw it. Everybody hated her and got a huge laugh out of it and the chocolate lube sat in the fridge for months :) Maybe put something like that randomly in their refrigerator or in a cabinet.

What we did at my friend's house was put among us stickers hidden all over the place. I even put one "inside" his monitor (behind the vesa mount) and I kept getting messages from him for MONTHS about where he found another one. There were stickers even in the inside of the lamps, in the trash can, under his keyboard and so much more lol

Everything gets funny then annoying then funny then impressive and then legendary when it's 5000 of something.

Googly eyes on everything!

Photos of their workspace, home, car. No one in the shot. Random photos of their things. Then attach them to the things. Photo of the microwave taped under microwave or on the side they dont normally see. Fridge pic on top of fridge. Photo of remote in where the remote batteries go.

Maybe label all the photos in a foreign language like Russian or something.

Something is odd when it's out of place and not easily dismissed. Developing a theme of some sort which infers some seemingly random items are somehow connected would add a sense of intention.

If your friend is in to fishing then you could litter fishing related things about their home. If they're into a certain spots team then leaving a rival team's merch would certainly catch their attention. You get the idea.

If you want to concoct something more elaborate and strange pick an object to start with and think of the most unrelated thing you could still associate with it. Like an eagle feather leading to Xavier: Renegade Angel. Going on a wiki-walk is great for this sort of brainstorming. Identify a bunch of loosely related but inexpensive items that are easy to hide and simple to find.

The "doubles of stuff they already have" bit is gold though.

Pancakes ... In there silverware drawer... In the sock drawer...in the bag of dishwasher tablets...pancakes everywhere

Just don't use syrup cause then it causes too much of a pain to clean when they find them.

Wait... Even better just Hershey kisses or mini peanut butter cups. Get a giant bag for like 20 and spread them around. They are individually wrapped so no mess and the first few times they will be happy for a treat but like after 50 they will be going crazy... Not safe if they have dogs but mostly harmless otherwise

Cabbages.

A friend of mine always hides a copy of ninja giaden z in our mutual friends house whenever we visit

Get an electric metronome (for playing instruments), unscrew a vent, turn it on, and screw the vent back on. He probably won't hear it till he goes to bed, then it's like a clock with a megaphone through the entire house.

You need the following:

• Plastic pipes
• end caps
• wool yarn
• table salt
• citric acid
• soy lecithin
• shampoo
• alarm clock
• popcorn

Leave all of it scattered on the table and let your friend's imagination run wild.

Price tags. In Euros or rubles. And signs in Polish, Swedish, Russian, etc. On all his furniture and appliances, possibly on each bedroom too

The friend who helped me move into this apartment ~10 years ago hid a ton of index cards folded in half with a joke on the outside and the punch line inside scattered everywhere. I was finding them for literal years. In my heavy winter jacket pocket (I moved very far south from very far north in the US so I didn't find that one 'til the one snow of the winter), in my board games, under my computer mouse, in my cooking pots.

Rubber ducks. Watch the video https://share.google/BUzibjkS7hJQ099w4 first.

My daughter once shared that video with me for laughs. Then she moved into her shared student accomodation. We helped moving in, and just before we left, I went to the bathroom. With a bag of ducks in my pocket.

I fondly remember her screaming "PAPA!" 😃

If they have a cat, take some brown clay and sculpt an unusually large cat shit and leave it in the litter box.

Leave a pineapple in the dishwasher or under the bathroom sink

Empty pringles can, 2 sponges, rubber gloves, and the biggest thing of lube you can find.

If they have a cat leave a dog toy, if they have a dog leave a cat collar etc.

Please edit your post so that it forms a question, as per the community rules.

If they have a cat … take a shit in the litter box.

Cricket chirpers

Get a litter box and fill it with kitty litter than take a massive dump in it.