Name a key which is hardest to turn.

What work can one never finish?

Look at the below sequence of letters and tell us what the next letter should be:

O T T F F S S ?

As shown in the image, the nine Dogs are square fenced. By constructing just two square fences can you make sure that two Dogs cannot meet each other without crossing the fence?

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 forty elephants and they have forty-fore heads. How can this be possible?

A tree doubled in height each year until it reached its maximum height over the course of ten years. How many years did it take for the tree to reach half its maximum height?

Decode below three cipher riddles:

26 L of the A
7 D of the W
1000 Y in a M

What will be the best approach to finding all the prime numbers less than 75 that leave an odd reminder when we divide them with 5?

What does this simple rebus mean?