$\textbf{Split the Coins}$
There are $100$ regular coins lying flat on a table. Among these coins, $10$ are heads up and $90$ are tails up. You are blindfolded and can not feel, see or in any other way to find out which $10$ are heads up. Is it possible to split the coins into two piles so there are equal numbers of heads-up coins in each pile?
$\textbf{Solution}$
It is possible. One way is to take out any $10$ coins to create one pile and make the remaining $90$ coins the second pile. Then, flip all the $10$ coins in the first pile. The job is done.
$\textbf{Analysis}$
This seemingly magic solution has a simple math behind it. Let the number of heads-up coins in the first pile be $x$ before they are flipped. Then, there must be $(10-x)$ heads-up coins left in the second pile. After having flipped the $10$ coins in the first pile, all the heads-up coins become heads-down and vice versa. Therefore, the $x$ heads-up coins will become heads-down and the $(10-x)$ originally heads-down coins will now become heads up. Now, both piles have $(10-x)$ heads-up coins.
$\textbf{Note}$
This is a good example of simple math enabled magic. Meanwhile, it is important not to unconsciously impose additional assumptions or constraints when solving brain teasers. For example, this problem neither requires the two piles to have the same number of coins, nor forbids coins from being flipped.