Which number should replace the question mark?

It is always ahead of me yet I can never see it. What is it?

The host of a game show offers the guest a choice of three doors. Behind one is an expensive car, but behind the other two are goats.
After you have chosen one door, he reveals one of the other two doors behind which is a goat (he wouldn't reveal a car).

Now he gives you the chance to switch to the other unrevealed door or stay at your initial choice. You will then get what is behind that door.

You cannot hear the goats from behind the doors, or in any way know which door has the prize.

Should you stay, or switch, or doesn't it matter?

There are three houses in a straight row. One is red, one is blue, and one is white. The red house is left of the middle. The blue house is right of the middle. Where's the white house?

Prove that the below maths equation is true.

8 + 8 = 91

Can you find out a way through which you can make five squares out of the given figure by moving just six match sticks?

What has many teeth, but cannot bite?

After teaching his class all about Roman numerals (X = 10, IX=9 and so on) the teacher asked his class to draw a single continuous line and turn IX into 6. The teacher's only stipulation was that the pen could not be lifted from the paper until the line was complete.

Is the statement valid that it does not matter how much older a sibling is, eventually the younger one will be half as old as the older one?

Why are all black people so quick ?