A useful application of this is Mishra’s Factory, which has three abilities: (1) a mana ability, (2) an “animation” ability, and (3) a “pump” ability. You can use the Factory’s second ability to “animate” it in the declare attackers step, block with it, and then tap the Factory with its third ability to pump itself in the declare blockers step! The end result is that your Factory is now tapped, but the game doesn’t mind; it’s still a blocking creature and still deals a pumped-up 3 points of combat damage to whatever it’s blocking (and it can survive 2 damage without dying!).
There is one catch, though!
One of the first rules new Magic players learn is the “summoning sickness” rule, which prevents creatures from attacking or activating “tap” abilities until their controller has controlled them continuously since the beginning of his or her most recent turn. This rule applies only to creatures, and not to other types of permanents (like lands). But if a non-creature permanent turns into a creature, the summoning sickness rule starts to apply in full. The Factory doesn’t get a pass just because it’s a creature and something else.
In other words, you can’t use the trick described above if you played the Factory on your last turn, since it will still be “sick.” You can still animate it and block with it, since even “sick” creatures can block–it’ll just have to block as a 2/2 instead of a 3/3. If you’ve controlled the Factory since the start of your most recent turn, though, you’re in the clear for a 3/3. The summoning sickness rule doesn’t care about how long it’s been a creature, only about how long you’ve controlled it.
Bonus tip: It’s impossible to use mana from one Factory to animate itself and then block, since the newly-animated Factory’s mana ability will have caused it to become tapped before your declare blockers step rolls around.
Today’s Rules Tip written by Paul Baranay