What's blue and bad for your teeth?
Answer
A really, really fast brick!
Explanation
The Doppler shift for fast moving objects moving towards the observer makes them appear blue due to the shortening light wavelength.
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That is an anti-joke, not a dad joke. But it is funny and tangential. I tell it frequently. Another:
What's blue and tastes like red paint?
Blue paint.
How often are bricks actually red?
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