There are 2 sand hourglasses.
The small one can measure 5 hours and the large one can measure 7 hours.
How can we measure 16 hours with 2 sand hourglasses running together ?
Can you arrange four 9's and use at most 2 math symbols, to make the total 100?
I am a three digit number.
My tens digit is five more than my ones digit.
My hundreds digit is eight less than my tens digit.
What number am I?
An express train takes 3 seconds to enter the tunnel which is 1 km long.
If it is travelling at 120 km an hour, how long will it take to pass completely through the tunnel?
2 ladies and 2 girls need to cross a river in a boat big enough for 1 LADY or 2 girls. How do they do it?
A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?
I have branches, but no fruit, trunk or leaves. What am I?
Look at the below sequence of letters and tell us what the next letter should be:
O T T F F S S ?
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?
You knock a Table over in the library, and suddenly 20 eyes are looking at you.
How do you go from 98 to 720 using just one letter?
There is a cryptic organization called Cicada 3301 that posts challenging puzzles online, possibly to recruit codebreakers and linguists.