Keeping your arm straight in front of you, you rotate 90° to your left, and see the left side of the circle lit while the right side is dark. Half the ball is still lit up, but you can see only part of the lit area. As you continue to rotate, you see a different amount of the ball.
You can explain how the moon changes in a pattern using models. Suppose you were standing in the dark room with a bare light bulb behind you. You hold a ball in front of you, and you can see all of the lit half of the ball, which looks like a circle. Keeping your arm straight in front of you, you rotate 90° to your left, and see the left side of the circle lit while the right side is dark. In fact, you would see the shape change from a full circle to a crescent shape to a backwards crescent shape and then back to a full circle.
