Can you find the key in the picture below?

I come in different shapes and sizes.
Parts of me are curved, other parts are straight.
You can put me anywhere you like,
but there is only one right place for me.
What am I?

A man and a dog were going down the street. The man rode, yet walked. What was the dog’s name?

13 decks of cards have been mixed. What is the minimum number of cards that must be taken out from the above-mixed cards to guarantee at least one 'four of a kind?

Accidentally, two trains are running in the opposite direction and enter a tunnel that is 200 miles long. A supersonic bird that has fled the lab and taken shelter in the tunnel starts flying from one train towards the other at a speed of 1000 mph. As soon as it reaches the second train, he starts flying back to avoid collision and meets the first train again at the other end. The bird keeps flying to and fro till the trains collide with each other.

What is the total distance that the supersonic bird has traveled till the trains collided?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

Replace the question mark with the correct number.

2 + 2= 4
3 + 3= 18
4 + 4= 48
5 + 5= ?

Can you find out the remainder when 3^300 is divided by 5?

There's one "sport" in which neither the spectators nor the participants know the score or the leader until the contest ends.

What is it?

When my father was 31 I was 8. Now he is twice as old as me. How old am I?