You're on fire today

Butane is a bastard gas.

A fisherman is sitting in his boat and decides to smoke, only to discover he didn't bring any matches.

He throws a cigarette overboard, and the boat becomes a cigarette lighter.

I love that the joke answer aligns with the physical answer here. Water weighs around 8 lbs per gallon while butane weighs about 5 lbs per gallon.

Butane does has a specific gravity of 0.601 vs water's 1.0

Nothing can be Everything is heavier than a Bic. No matter what, the Bic is always a little lighter.

Shoulda gone with "pound" or "kilo" since "gallons" of different things often weigh different amounts.

Shouldn't it be "which weighs less?"

You could skip "Water" and just hit the punchline. "Butane is a lighter fluid" answers the question.