“Summoning Sickness” is a common phrase to describe a much wordier rule: A creature cannot attack or activate abilities with the Tap or Untap symbol unless it has been under your control continuously since the beginning of your most recent turn (unless it has haste). This is usually pretty clear-cut. For example, you can’t tap Llanowar Elves for mana the same turn you cast it, and you can’t attack with it that turn either.

Note that the summoning sickness rule is very specific, however. It doesn’t stop a creature from using any other ability that doesn’t include the Tap or Untap symbol (e.g., Fume Spitter), and it doesn’t stop you from tapping that creature to pay the cost of a different spell or ability.

For example, let’s say you control a Skirsdag High Priest that you cast on your last turn, and a creature died during your attack step this turn. If you cast Lingering Souls to get two spirit tokens, you can tap the Priest and those two tokens to activate the ability. The Priest itself can’t have summoning sickness, because its ability has the Tap symbol; however, the other two creatures are simply tapped to pay the rest of the cost for the Priest, so their summoning sickness doesn’t matter. Similarly, you can tap Grand Architect (or any other blue creature that has summoning sickness) to activate its ability the same turn it enters, because the ability doesn’t actually include the Tap symbol.

Today’s Rules Tip written by
Josh Stansfield, Level 2 Judge from Orange, CA