What Adam and Eve do not have but the rest of the people have?

Complete the series by replacing "?" with the correct number.

ST ND RD TH '?'

John and Jenni are a married couple. They have two kids, one of them is a girl. Assume safely that the probability of each gender is 1/2.
What is the probability that the other kid is also a girl?

Hint: It is not 1/2 as you would first think.

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

If you toss a coin 10 times and it lands heads up every time, what are the chances it will land heads up if you toss it again?

Nine marbles are arranged in the image below. Can you slide two marbles to form a square?

Is the statement valid that it does not matter how much older a sibling is, eventually the younger one will be half as old as the older one?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?

Can you replace the X (both small and big x) with the numbers between 1 to 19 such that all the rows of 3 numbers between X (big x) sum to 23?

I had an infinite supply of water and 5 litres and 3 litres jars.

How would you measure exactly 4 litres in the least number of steps?