Probability Question

It is interesting to interview fresh grad candidates. I'll post interesting questions as I find them. Please let me know if you have good solutions. I often find the candidates in interviews cannot answer the following question correctly.

A couple has two children. One child is boy. What is the probability that the other child is boy ? Assume that a boy or girl child is born with probability 1/2.

Another interesting version: A couple has two children. One of them is a boy who is born on Sunday. What is the probability that the other child is a boy. Assume that a boy or girl child is born with probability 1/2.