It's a Small World

All Kithkin, Halfling, Dwarf, and Gnome creatures, lose their creature types, and become Bitty creatures instead.

Whenever a bitty creature enters the battlefield under your control, you may choose another bitty creature in your hand, and put it onto the battlefield tapped. If you do, sacrifice another creature you control.

This ability triggers only if a creature you control left the battlefield this turn.