We 

have 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up and rest of 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. 

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Answer:

Divide all 100 coins into two piles. (irrespective of pile-size)

And flip all the coins in any one pile.

which gives equal number of heads and tails in both piles.