Riddle Of The Weights

There are two boxes. They both weigh the same.
The contents of the boxes are swapped round.
Now the boxes both weigh differently.
Why? (Scroll down to read the answer).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Answer:
One box was a light box containing a heavy weight. The other box was a heavy box containing a light weight. When the weights inside were switched round, the boxes became a light box containing a light weight and a heavy box containing a heavy weight.