The professor  showed a large cage with a male rat in it. The rat was in the  middle of the cage. Then, the professor placed 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.

The Professor then 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.

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!!!”