The professor showed a large cage with a male rat in it.
The rat was in the middle of the cage.
Then, the professor kept a piece of cake on one side and kept a female rat on the other side.
The male rat ran towards the cake and ate it.
Then, the professor changed the cake and replaced it with some bread.
The male rat ran towards the bread.
This experiment went on with the professor changing the food every time.
And, every time, the male rat ran towards the food item and never towards the female rat.