When they walk on the ground, their heads bob back and forth with each small step.

Their heads bob up and down all at about the same height.

A long way ahead now he could just see the top of her head bobbing along.

His big head bobbed up and down, but he didn't look back.

The black man's head bobbed up and down in agreement.

His head bobbed between them with the effort of memory.

Her head bobbed back and forth as she tried to see.

His head and shoulders bobbed as he pulled himself to the door.

Now the first soldier appeared, a head bobbing near the ground, coming toward him.

His head bobbed up and down as he followed the beat of the song.