If your son goes to an all-boys school, are his classmates more likely to have a brother or a sister? Steve's son goes to an all-boys secondary school and Steve is going to collect him from school after his first day of school.
Steve does not know any other parent yet and meets Richard at the gates, who also has a son in the same class. Richard tells Steve that he also has another child.
What is the probability that Richard's other child is also a boy?
Note: this is to try to better understand the Boy-girl paradox (https://en.wikipedia.org/wiki/Boy_or_Girl_paradox), which was pointed out to me in an anwser to one of my earlier questions
 A: With all of the reasonable necessary assumptions (equal probability of boy or girl, probability of sending one child to an all boy's school independent from gender to other child......) the probability that Richard's other child is also a boy is (drum-roll)
$$
\frac{1}{2}.
$$
When you hear these problems a good thing to think is "can I label them in such a way that I know the gender of one of the labeled children?"  In this case I will say the child in Steve's sons class is child A and the other child is child B.  For simplicity later on I'll refer to boy as 1 and girl as 0.  Our assumptions boil down to:
$$
P(A=x,B=y) = P(A=x) \cdot P(B=y) = \frac{1}{2} \cdot \frac{1}{2} = \frac{1}{4}.
$$
The question you are asking concerns $P(B=1 | A=1)$.  By independence or direct computation we will easily get $1/2$.  
The paradox focuses on the scenario where I know someone has two children, and I know they have at least one boy.  No matter how I try to label the children, (youngest=A, oldest=B) (shortest=A, tallest=B) I don't know the gender of one of the labelled children.  Notice I can not talk about the gender of "the other child" because I don't know of one specific child who is a boy.  This is quite different than the other problem.  Once i pick some arbitrary labelling, the quantity of interest mathematically is 
$$
P(A+B=2 | A+B\geq 1) = \frac{1}{3}.
$$
A: It's 50% (or whatever the actual $p$ of a male vs female set is in the population drawn from, but 50% for simplicity.)
We know for a fact that this concrete child is a boy, so the only open situation is the gender (sex?) of the other child.
Another way to frame this particular question is:
"There is a child of unknown gender. What is the probability that the child is male?"
To illustrate the paradox, it would be better to examine a question like this:

From the families with exactly two children and at least one boy, what is the probability that
  one of these families chosen at random has two boys?

A: The probability is 1/2 cause P(girl|boy)=P(girl) cause they are independent and the fact that Richard has a boy doesn't give any useful information about the other child.
