I am white when I am dirty, and black when I am clean. What am I?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

On my way to St. Ives I saw a man with 7 wives. Each wife had 7 sacks. Each sack had 7 cats. Each cat had 7 kittens. Kittens, cats, sacks, wives. How many were going to St. Ives?

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see? What is it?

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

How high do you have to count before you use the letter “a” in the English spelling of the whole number?

I never was there, I am always to be.
You have never ever seen me, not ever you will.
Yet I am the confidence of everyone.

Who Am I?

What is the last thing you take off before bed?

Two friends were stuck in a cottage. They had nothing to do and thus they started playing cards. Suddenly the power went off and Friend 1 inverted the position of 15 cards in the normal deck of 52 cards and shuffled it. Now he asked Friend 2 to divide the cards into two piles (need not be equal) with equal number of cards facing up. The room was quite dark and Friend 2 could not see the cards. He thinks for a while and then divides the cards in two piles.

On checking, the count of cards facing up is same in both the piles. How could Friend 2 have done it ?

Is the statement valid that it does not matter how much older a sibling is, eventually the younger one will be half as old as the older one?