Urza, Academy Headmaster

If an ability comes up that requires a target and there are legal targets available, you must use that ability even if you may not want to. However, the effects are all good for you or bad for your opponent. In some cases, you may not be able to take advantage. For example, if it ends up that creatures you control get +1/+1 until end of turn but you don't control any creatures, you're out of luck. But nothing bad will happen.

Players can respond to the activation of any of Urza's abilities, but not after players know what effect is going to happen.

Urza's abilities work a little differently with respect to targeting. Unlike other abilities, you may have to choose the targets for an ability as part of that ability resolving. If the ability that comes up requires a target and there are no legal targets available, click again until that's not true.

Urza, Academy Headmaster is just like any other planeswalker, except you're never quite sure what his abilities are going to do. Urza is very unpredictable. To find out what happens, head to AskUrza.com. There you will find three buttons: +1, -1, and -6. Click the button for the loyalty ability that's resolving, and follow whatever instructions appear.