What the third seat's for

I let Lily crawl around cab of my truck, since it was safer than having her wait in the parking lot as Nate was buckled into his car seat.   She hopped over the front seats into the back - 2/3rd's of which are covered in car seats.    She asked what the third seat in the back of the truck was for, and I said it was there in case of any accidents.  

She didn't get the joke.