When it's bad luck to meet a black cat?

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

A train full of passengers goes through a tunnel. When the train comes out from the other end, there is no single person on the train.

How did it happen?

A boy was locked in a room by some robbers. All that is in the room is a piano, a calendar, and a bed. The room is locked from the outside. What does he eat, drink, and how does he escape and get out?

Can you find the missing piece of the Porsche?

You are in a room with no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify which one is a magnet?

There are are five things wrong with this sentence; only geniuses will be able to to spot all of the mitstakes

What goes up but never comes down?

I know a ten-letter word in the English language which can be typed using only the top rows of the computer keyboard.

I am a type of food that is often found in the fridge and can be white in colour. What I am?
Hint 1: I am a dairy product.
Hint 2: You can spread me on the bread.