The rain is initially a circle.
Click the [left mouse button] to start a rain.
The center of the rain is where you have clicked.
To adjust the radius of a rain, hold the [left mouse button] and move the mouse. A red circle will appear. When you release the left button, the circle becomes blue and the rain stars.
Once you have created a rain, you cannot change it or stop it.
You can have multiple rains.
fix variable names
fix rain simulation bugs
explain interface
add fullscreen feature