What does this rebus mean?

O
N
C
E
TIME

If you read clockwise, you can form a word by inserting three missing letters in the picture given below. Can you do it?

A frog is at the bottom of a 30-meter well. Each day he summons enough energy for one 3-meter leap up the well. Exhausted, he then hangs there for the rest of the day. At night, while he is asleep, he slips 2 meters backwards. How many days does it take him to escape from the well?

You board a train. The train will have to enter a tunnel soon. You are claustrophobic. Which place is the best for you to sit?

What has 88 keys?

Can you find out which triangle will have a bigger area among the following?
1. A triangle with sides 300, 400 and 500
2. A triangle with sides 300, 400 and 700

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 ?

What jumps when it walks and sits when it stands?

Can you complete the sequence?

192, 021, 222, 324, 252, 627, 2__, 9__?

What is the last thing you take off before bed?