Ok,
Let me explain the concept.
1> Take two coins of same type
2> Put them on top of each making sure that you dont see both heads or both tails.
3>...Now show these coins to someone from 5-6 foot distance, holding in hand such a way that only one side is visible.
4> Ask that person to guess how many coins are there
...Expected answer is "One"

5> Turn the set of coins around and repeated the question..
... Again "one".

6> Show the side view of these coins.
Point is.

>>>> Without looking at the 3rd side you cant separate both the side of coins.
>>>> Without looking at the 3rd side you wont realize if it is one coin or multiple.
>>>> Quite often there are more 2 opinions.... And by looking at the 3rd side you can identify that.
Also, Good point by gohm, 3rd side is where both side meet and makes a coin with value.
