Everyone in a circle.
One person (A) starts by calling another person's name (B).
At that point, A starts walking towards B.
Before A gets to B, B needs to call another name (C), so B frees her spot in the circle, for A to take.
C then needs to call yet another name and start walking, so B can take C's spot.
Variations
Instead of calling names, players can pick another player by making eye contact (and nodding to acknowledge that eye contact has been made). This demands a lot of concentration.