What is the cause of bright yellow liquid discharge from nose

My 20 YR OLD SON called me from university and told me that he had a bright yellow liquid discharge from his nose. He thought(felt like) his nose was bleeding but when wiped his nose he saw that it was a bright watery yellow. When he was a child he suffered from nose bleeds but at least 8 years if not more he hasn't had them. He did not suffer any injury in his head or elsewhere. He says he feels healthy. This incident happened yesterday once and today twice, when he was sitting in class not touching his nose