I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

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

I have teeth but i don't bite. Who Am I ?

I am taken from a mine and shut up in a wooden case, from which I am never released, and yet I am used by almost every person. What am I?

John left on a horse on Thursday , was gone for two days, and came back on Thursday after completing his work. How did that happen?

What jumps when it walks and sits when it stands?

There are three boxes of different sizes with three cupcakes inside each of the boxes in Christina's kitchen. At night her daughter wakes up and goes to the kitchen. She opens a box and eats all three cakes.

But in the morning Christina finds out that each box still had three cupcakes. How?

You have three orange, two pink and five purple balls in the drawer beside your bed. There is no electricity and the room is entirely dark. How many balls must you take out to ensure at least one ball of each colour at least?

What jumps when it walks and sits when it stands?

Can you decipher these quotes by Albert Einstein?
Blf xzm mvevi hloev z kilyovn lm gsv ovevo lm dsrxs rg dzh xivzgvw.