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 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 kept 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.

Professor said:
This experiment shows that food is the greatest strength and attraction.

Then, one of the students from the back rows said:-

"Sir, why don't you change the female rat....?

She may be his wife!!"