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Nevermark
Just grab some paper, a pen, and check that no number equal or smaller than its square root divides into it evenly.

That is it. That is all. Pish posh.

IsTom
My favourite prime checking algorithm is that for n < 100 if it looks prime, it is prime.
floydian10
91 should be prime, ridiculous that it isn't
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sunrunner
I wonder what the underlying human intuition is for 'prime-ness' and why it might break down with larger numbers. Odd numbers in the rightmost position? The 'shape' of the number (phonaesthesia, the bouba/kiki effect)? Maybe they just sort of feel scary?
30030
That's easy.

(factorial(170,141,183,460,469,231,731,687,303,715,884,105,726) + 1)%(170,141,183,460,469,231,731,687,303,715,884,105,727) == 0

oliviersca
For those who enjoy burning cpu cycles ! m1277 = 2 ^ 1277 - 1 is not prime. It easy to check it with the Lucas-Lehmer test. But we don't know any of its divisors, which is quite fascinating.