Until it is measured, not known. Yet how you miss it when it has gone. What is it?

What is the word or phrase?

Can you find the missing letter in the sequence?
S - T - I - L - ? - T - F - Y - C

I’m tall when I’m young, and I’m short when I’m old. What am I?

Some friends went on a vacation to a resort. It was raining heavily and it kept raining for thirteen days.

When it rained in the morning, the afternoon was beautiful and when it rained in the afternoon, the day was blessed with a clear morning.

Overall, the friends experienced eleven nice mornings and twelve nice afternoons. Can you find out the number of days they spent on vacation?

What do you see in the image below

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.

I can be cracked, made, told, and played. What am I?

What runs around the whole yard without moving?

If eleven plus two equals one, what does nine plus five equal?