How many stars can you see in the morning?
Only one! The Sun!
An electric train is moving from North to South at 100km/h. The wind is blowing from South to North at 50km/h. At what speed does the train blow?
It's an ELECTRIC train. There's no smoke!
How many books with 30cm x 30cm can you put in a 1m x 1m box, so it's not empty anymore?
Just one! After that, the box is not empty anymore!