You then proceed to count 6 coins, one by one into their open hand. They then close their hand quickly and confidently call out the number of coins.

To their surprise, when they open their hand they find out their answer is wrong, allowing you to keep your money.

To their surprise, when they open their hand they find out their answer is 

wrong, allowing you to keep your money.
The Secret: Six coins are taken out from your pocket and shown to your spectator. They can clearly see six coins and so will be confident of winning the challenge.

Count the coins one by one into their open hand, counting aloud, "one" "two" "three" "four" but when you get to coin number five, you still say "five" but simply tap this coin against another coin in their hand but do not let go of the coin.

Finally announce "six" and drop coin number "six" into their hand, after which they need to close their hand quickly.

This is an easy to do coin trick which can be good fun. You can easily slip the fifth coin back into your pocket or make it appear from somewhere to further enhance the magic.