You know the pile of books in your front seat is probably too heavy

when your car thinks you should fasten a seat belt around it.

(Translation: I checked out a pile of GED prep books for the ESL night class I'm currently teaching. I set it in the front passenger seat of my car, and got ready to drive off. I noticed an annoying flashing light; it's a symbol of a person wearing a seat belt. The car has noticed that my passenger-seat poundage has exceeded a certain amount, has concluded that I have a person sitting there, and wants me to follow the law and have them buckle up.)