What always comes into a house through the keyhole?

Count the number of triangles in the picture below:

What is black when you buy it, red when you use it, and gray when you throw it away?

What is the end of everything?

A plane crashed between the border of Mexico and America. Where do you bury the survivors?

Can you find out the missing number?

? 4 5 3 5

9 5 7 10 3

You measure my life in hours
I serve you by expiring
I vanish faster when I am thin
and slower when I am thick
The wind is my energy
What am I?

Assume there are approximately 5,000,000,000 (5 billion) people on Earth. What would you estimate to be the result, if you multiply together the number of fingers on every person's left hand? (For the purposes of this exercise, thumbs count as fingers, for five fingers per hand.) If you cannot estimate the number then try to guess how long the number would be.

People make me, save me, change me, raise me. What am I?

What does below science rebus mean?

Ag Ag Ag
Ag SPOON Ag
Ag Ag Ag