You have a 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up, 90 are tails up. you can’t feel, see or in any other way find out which side is up. split the coins into two piles such that there are the same number of heads in each pile.

I LOVE THIS PUZZLE

so,
the solution is
take 10 coins off to the side
flip those 10 over
done

sounds too simple?
 

this can be solved with algebra for ANY number of coins
lets say we have 50 coins, all tails
we flip h coins head
if h=x+y
if we seperate h coins off, we get x heads, and y tails
and in the other pile, we have y heads and the rest of the tails
so now, if we flip all the coins, we get y heads  in the pile and x tails

solution:
seperate 10 coins and flip all 10 of them over