No Rent

A Story Riddle

An ant knocks on the door of a house. 

The house owner opens the door.

"I want a place to stay," said the ant.

"I have a vacant room which you can occupy for free of cost," said
the owner. 

The ant went inside and occupied the vacant room.

After some days, the ant brought in another ant and requested to the
owner, "Can you please allow this ant to stay with me?"

"Oh sure, you can do so without paying any rent," said the owner.

After some days the ant brought one more ant and requested the owner
to allow the ant to stay with it. 

The owner agreed to it without asking for any rent.

This continued as the ant brings in one more and and the owner
agrees for it. 

One fine day, the ant brought in a tenth ant and requested the owner
to allow him also to stay with it.

The owner said, "OK, you can all stay here but you all need to pay
rent."

Now the question is: Why did the owner ask for rent when the last
ant came in?

Because they are now tenants!