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

Can you find out which number multiplied by itself will give the output as 12345678987654321?

What animal walks on four legs in the morning, two legs during the day, and three legs in the evening?

Oedipus, the king of Thebes, figured out the answer to the riddle:

How many words can a pen with half refill write?

1 + 9 + 8 = ?
considering, 28 + 8 + 92 = 10

Two old friends, Jack and Bill, meet after a long time.

Three kids
Jack: Hey, how are you, man?
Bill: Not bad, got married and I have three kids now.
Jack: That's awesome. How old are they?
Bill: The product of their ages is 72 and the sum of their ages is the same as your birth date.
Jack: Cool..But I still don't know.
Bill: My eldest kid just started taking piano lessons.
Jack: Oh, now I get it.

How old are Bill's kids?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

Can you complete the sequence puzzle?

AR TA GE CA LE VI LI _

Find out the number of circles in the given picture having Illusion

A sundial has the fewest moving parts of any timepiece. Which has the most?