What happened when the wheel was invented?

Can you think of any three-dimensional shape that comprises of exactly two surfaces?

What occurs twice in a week, once in a year but never in a day?

What word begins and ends with an E, but has only one letter?

What does this mathematical rebus means ?

Replace the question mark with the number below given a simple math problem.

A train full of passengers goes through a tunnel. When the train comes out from the other end, there is no single person on the train.

How did it happen?

After teaching his class all about Roman numerals (X = 10, IX=9 and so on) the teacher asked his class to draw a single continuous line and turn IX into 6. The teacher's only stipulation was that the pen could not be lifted from the paper until the line was complete.

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

Can you find the next number in the below sequence
3 , 7 , 10 , 11 , 12 , 17 ?