The program I am designing has a wizard shooting a thunderbolt or fireball (depending on the button pushed) at a knight. I have the scene set up with the fireball and thunderbolt attached to the wizards hand and they are both using opacity 0. What I need to know is how to make it become visible at the moment that the key is pushed. I can make it move, but I don't know how to change it's opacity when the key is pushed.

I am not good at explaining this kind of thing so if you need me to explain it in another way let me know. If anybody knows how to do this please let me know.