Duck

Level Up (You may pay {x} at any time to level up
this creature, where {x} is its level plus 1. Also it
starts on level 1.)
Level 1 - {t} Sacrifice Duck: whenever a creature
enters the battlefield give it +0 +1 (This stacks.)
Level 2 - {t} Sacrifice Duck: whenever a creature
enters the battlefield give it +0 +2 (This stacks.)
Level 3 - {t} Sacrifice Duck: whenever a creature
enters the battlefield give it +0 +3 (This stacks.)