Examples: Rockets
File: snow.py

Animate circles (snow) falling down from the sky and piling up on the ground.

Use an animation loop. Each time through the loop:

1. Animate all snow flakes not on the ground
2. Generate a new snowflake at the top of the screen with 10% probability, at a random horizontal location
3. Call update() on the canvas
4. Pause for 10 milliseconds