What does this rebus mean?

O
N
C
E
TIME

Can you count the number of seconds in a year?

Clue: You need to bother about calculation.

Can you find out a way through which you can make five squares out of the given figure by moving just six match sticks?

The following sentence is false. The preceding sentence is true.
Are these two sentences true or false?

A wooden log balances equally on the scales when weighed with three-quarters of a pound and three-quarters of a wooden log.

Can you calculate the weight of the entire wooden log?

Prove that the below maths equation is true.

8 + 8 = 91

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?

Your last good ping-pong ball fell into a narrow metal pipe embedded one foot deep in concrete.
How can you get it out undamaged, if all the tools you have are your tennis paddle, shoe laces, and plastic water bottle, which does not fit into the pipe?

John killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

How many letters are in the alphabet?