a hard or padded protective hat, various types of which are worn by soldiers, police officers, firefighters, motorcyclists, athletes, and others
Wear a helmet when you ride a bike.

a cover that can be lifted or removed
I put a lid on the jam.

a covering that protects or hides the face, such as for ceremonies
I put on a monster mask on my face.

a machine controlled by a computer that is able to move and perform many of the same tasks as people can
I have a toy robot.

the surface that forms the top or cover of a building, vehicle, or other structure
Santa Claus is on my roof!

thick thread
The kite is tied to a long string.

요요
I have a toy yo-yo.