Why Aeroplane windows are round?

We have four gears with below features=> Both Gear A and D has 60 teeth=> Gear B has 40 teeth=> Gear C has 20 teeth=> Every two minutes, Gear B makes 25 full turns.What is the relative speed of Gear A and Gear B?

When it's bad luck to meet a black cat?

When Jack was six years old he hammered a nail into his favourite tree to mark his height. Ten years later at age sixteen, Jack returned to see how much higher the nail was. If the tree grew by five centimetres each year, how much higher would the nail be?

I taste better than I can smell. Who am I?

Replace the question mark with the correct number.

They do not possess flesh or feathers or scales or bone
But still, they are blessed with fingers and thumbs of their own.

What does does this rebus picture means ?

How can you make number one disappear by adding something to it?

If in a code language, DISTANCE is written as IDTUBECN and DOCUMENT is written as ODDVNTNE, the how will THURSDAY be coded as in the same language?