What goes up but never comes down?

I had an infinite supply of water and 5 litres and 3 litres jars.

How would you measure exactly 4 litres in the least number of steps?

You are in a room that has three switches and a closed door. The switches control three light bulbs on the other side of the door. Once you open the door, you may never touch the switches again. How can you definitively tell which switch is connected to each of the light bulbs?

Krish is 45 years old and his mother Crishtina is 73 years old.

Can you find out how many years ago, his mother was three times his age?

A man has to get a fox, a chicken, and a sack of corn across a river.

He has a rowboat, and it can only carry him and one other thing.
If the fox and the chicken are left together, the fox will eat the chicken.
If the chicken and the corn are left together, the chicken will eat the corn.

How does the man do it?

What does this rebus picture identify?

John’s parents have three sons: Snap, Crackle, and what’s the name of the third son?

Can you count the number of quadrilaterals in the picture below?

Why is 6 afraid of 7?

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?