They must have a smooth surface so that the child does not hurt himself.

"Now, raise your hand if you think the model has a smooth surface."

She ran a hand along the smooth surface of the floor.

One entire wall of the room was taken up by a smooth black surface.

Door took the box and ran her fingers over the smooth surface.

He reached out, and his hand hit a smooth surface.

No need of a smooth surface since there isn't any contact.

Then he looked down at the smooth black surface of the water.

Her eyes stared over the smooth dark surface of the table.

Finally it led to a river, large enough to have a smooth surface.