Look closely at the two bottom sets of fins. Notice that the blades on both are angled in the same direction yet the bottom fins are turning in a counter clockwise direction while the set above it are turning clockwise. Can you guess why?

Answer: Both sets of fins are driven by moving air. The bottom set is moved by an up-draft from a fan underneath.

The set of fins just above it and turning in the clockwise direction are being driven by a down -draft caused when the air reaches the top of the glass and is deflected back down