猜謎語:什麽東西出去的時候是滿的,回來時是空的?

我先答兩個：

1.上班午餐飯盒；

2.家裏的垃圾桶;

3. Car's gas tank ---- When you drive out from gas station, it's full; when you come back again, it's empty.

(big-guy原創)