Positive space

  

Negative space

 

The above two structures are identical except that one is flipped upside down. There is some optical illusion element where our eyes play trick on our brain to interpret them differently.

The interesting thing is that these two objcet being positive and negative, they  fit nicely onto each other.

There is a page dedicated to how to build this type of structures if you want to find out more.

 

More examples: