Why does the hammer release when I turn on the safety of my ****** handgun?

It's a smith and wesson 5904. I have not loaded it yet because I am not sure what is going on here: When I it the hammer get's pulled down as if it will fire. I can then pull the trigger and it will (I think) fire. However if I it, and then flip the safety on, the hammer closes the same way it does when I pull the trigger. I don't want to load the gun, and have it go off when I click the safety on. What's going on here?