About ants and rentals

Here's a nice joke (thanks to Fr Koshy for passing it on):

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 more 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!