Materials:

A stick of incense

A lighter or matches

A grown-up to light it for you

Procedure: Light the incense stick and let it burn. As long as you don’t move the stick, and there’s no wind, you’ll notice the smoke rising in a straight, narrow plume and then, suddenly, changing into a tangle of smoke.

Explanation: The hot smoke is more buoyant than the surrounding air. This buoyancy accelerates the smoke upward. Initially, the smoke has low velocity and flows smoothly. As the velocity reaches some critical value, the flow becomes turbulent.