When Frog Gender Flips

Frogs that are male acting like female frogs that sounds impossible doesn't it. Well its true a California college students was in the lab and saw thaqt the frogs where acting different. For the next few days his boss told him to observe the frogs. The students observed the frogs and found out that something in the water was making them change. What I dont understand is how water makes something change what it does. How does everything happen to animals and nothing happens to humans.