A prank I recommend to readers is to use the number 91 when a group situation calls for a random prime number. If done subtly enough, a decent portion of mathematicians will believe you. Granted, its not a particularly funny prank…
So why 91? Look at the main ways to quickly check primality of two digit numbers:
Is it even?
Does it end in 5?
Do its digits add up to a multiple of 3?
Are the two digits the same?
Is it a square?
For these, 91 is the only composite number that all of them miss (being 7 times 13). Most mathematicians run those checks in their head, or (more likely) they are pretty good at spotting immediately numbers which fail the above checks. Anything that passes these checks at least ‘smells’ prime.
This fact is also useful for something other than lame pranks. It means that you can know for sure which two digit numbers are prime, simply by adding one last check to the list:
Is it 91?
Its not super useful, but it does cut down on the amount of time it takes to check numbers like 83. It smells prime, and its not 91, so its prime! Its like the quickest Sieve method imaginable.