One time I told the twins to ask the car if it needs gas, so they said:
"Car, you meed gas?" (they can't say "need" yet)
And I answered as the car (with a weird squeaky car voice, don't picture KITT from Knight Rider, you'd be way off).
Now every time we pass a gas station, it's "Car, you meed gas?" and they won't take no for an answer. I've started filling the tank when it's only half empty, just so the twins get the chance to help me more often.