Urza, Lord High Artificer (Future Sight) - Mystery Booster 2 (MB2)

When Urza, Lord High Artificer enters the battlefield, create a 0/0 colorless Construct artifact creature token with "This creature gets +1/+1 for each artifact you control."
Tap an untapped artifact you control: Add U.
5: Shuffle your library, then exile the top card. Until end of turn, you may play that card without paying its mana cost.