>>>A soldier was ordered to go and shave all the soldiers in his squad - and only those - who don't shave themselves.
>>>
>>>After thinking it over for a while, he discovered a fundamental flaw in the command, which made it impossible for him to carry out the order. What might that be?
>>
>>Him.
>
>Yes, that is right.
>
>Variations of this problem are often cited in Set Theory.

I liked it.