Rulings

• 2007-10-01
  Activating the ability with X = 0 will cause all creatures you control to become 0/0 and be put into the graveyard.
• 2009-10-01
  You apply power/toughness changing effects in a series of sublayers in the following order: (a) effects from characteristic-defining abilities; (b) effects that set power and/or toughness to a specific number or value; (c) effects that modify power and/or toughness but don’t set power and/or toughness to a specific number or value; (d) changes from counters; (e) effects that switch a creature’s power and toughness. This card’s effect is always applied in (b), which means that effects applied in sublayer (c), (d), or (e) will not be overwritten; they will be applied to the new value.
• 2013-07-01
  Examples of creature types include Sliver, Goblin, and Soldier. Creature types appear after the dash on the type line of creatures.
• 2007-10-01
  Activating the ability for a second time in the same turn will overwrite the first activation. For example, if you activate the ability with X = 4, then activate it with X = 1, each of your creatures will be 1/1.
• 2021-03-19
  Mirror Entity’s ability overwrites any effects that previously set a creature’s base power and/or toughness. Any existing effects or counters that raise or lower a creature’s power and/or toughness continue to apply to the creature’s newly-set power and toughness.
• 2021-03-19
  Changeling applies in all zones, not just the battlefield.