Algodoo

Here's a quiz for you to solve. Explain how the circle is turning, although the box that it's atached to is stationary. The velocity readout on the box is not fake, and you can test it by grabbing the box with the grab tool and sliding it back and forth. You will note the velocity readout increase (positive and negative) as you slide it back and forth. Also, you can look at the parameters for the axel (hinge) to see that it's not configured as a motor! So, tell me in the comments what makes the circle turn.

NOTE: THis quiz is not for experts (like Kilinich) because it's too easy for those guys. It's mainly for beginner Algodooers.