The middle ability can be activated only once. Not once per turn. Once. Okay, maybe more times. If Urza's Fun House leaves the battlefield and returns, it's a new object, so you can activate it again. You know, in case INFINITE MANA wasn't enough the first time.

The mana added by the middle ability is colorless mana.

Once you have infinite colorless mana, it doesn't matter how much mana you spend, you will still have infinite colorless mana. You will, however, lose infinite unspent mana as that step or phase ends.

You can do a lot of cool stuff with infinite mana. And some situations may not be obvious. Rather than rehash everything ad infinitum, perhaps the Gatherer rulings on Infinity Elemental will help.

The Urza's Mine, Urza's Power-Plant, and Urza's Tower required to activate the middle ability needn't be three separate permanents. If you happen to control a land that has those four land types (or perhaps one that has all land types . . . you know . . . glances up . . . it's a different card in this . . . Nearby Planet!), you can activate the ability.

AskUrza.com, the web site that powers Urza, Academy Headmaster, has expanded to support Urza's Fun House. On that site you'll find a button that says "ENTER THE FUN HOUSE." As the last ability resolves, click it and find out what you paid seven mana for!

The last ability works a little differently with respect to targeting. Unlike other abilities, you may have to choose targets while that ability is resolving. If the effect that comes up requires a target and there are no legal targets available, click again until that's not true.

If an effect comes up that requires a target and there are legal targets available, you must use that effect even though 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 +3/+3 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 the last ability, but not after players know what effect is going to happen.