White sands

Walk along a beach at low tide when the sand is firm and wet. At each step, the sand immediately around your foot dries out and turns white. Why? The popular answer, that your weight ‘squeezes the water out,” is incorrect: sand doesn’t behave like a sponge. So what does cause the whitening?

Click here for the solution

Before you step on it. the sand is packed as tightly as it can be under natural conditions. Your weight disturbs the sand, making the grains less efficiently packed. The sand is forced to occupy more volume and rises above the water level. becoming dry and white. The water rises more slowly. by capillary action, so it takes a few seconds or more before the sand gets wet and dark again.

more riddles

How can this be?

A certain large animal lives happily and thrives here on Earth. One day, every single one of these critters is wiped out by a mysterious disease which affects only this particular animal. There are none left anywhere on earth -- they are all gone. About a year or so...

read more

How old is Mike?

David and Mike are brothers. David is 33 years old today. This is three times as old as Mike was when David was the age that Mike is today.How old is Mike?Mike is 22 years old.more riddlesDonec rutrum congue leo eget...

read more

How many – how much?

The engineering department was arranging for a rather expensive catered lunch to bid farewell to their retiring colleague. They calculated that it would cost each person $30. One good mathematician remarked, "It's lucky that there aren't five fewer of us to split the...

read more

Which ball?

Two barrels contain each the same amount of water. The water temperature in one barrel is 49°F while the other is at 29°F. Two golf balls of same dimension and weight are dropped simultaneously into the barrels. The golf balls are dropped from the same height.Which...

read more