Version 8
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There is a world with 1 randomly-placed, randomly-sized,
stationary particle. There are buttons to play, step, and
reset the model. Stepping the model causes the particle to
move. If the particle hits a wall, it bounces off. 
Resetting the model places the particle at a new
random location with a new random size. Playing the model
causes it to step repeatedly. Resetting the model causes it
to stop playing. Once the play button has been clicked, it
can't be clicked again until the model is reset.
1. Add an "if" statement to the step function that checks if the particle
has gone past a wall, and if it has, reverses the direction of the particle
2. Anywhere there is code for center_x and velocity_x, add code for center_y
and velocity_y

Version 7
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There is a world with 1 randomly-placed, randomly-sized,
stationary particle. There are buttons to play, step, and
reset the model. Stepping the model causes the particle to
move. Resetting the model places the particle at a new
random location with a new random size. Playing the model
causes it to step repeatedly. Resetting the model causes it
to stop playing. Once the play button has been clicked, it
can't be clicked again until the model is reset.
1. In the reset function, call clearInterval
2. In the play function, add a line to disable the play button
3. In the reset function, add a line to enable the play button

Version 6
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There is a world with 1 randomly-placed, randomly-sized,
stationary particle. There are buttons to play, step, and
reset the model. Stepping the model causes the particle to
move. Resetting the model places the particle at a new
random location with a new random size. Playing the model
causes it to step repeatedly.
1. Add an HTML button element for the Play button
2. define a JavaScript function that is called whenever the button is clicked
2a. the function will call the step function repeatedly using setInterval

Version 5
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There is a world with 1 randomly-placed, randomly-sized, stationary particle.
There are buttons to step and reset the model. Stepping the model causes the
particle to move. Resetting the model places the particle at a new random
location with a new random size.
1. Add an HTML button element for the reset button
2. Define a JavaScript function that is called whenever the button is clicked
2a. The function will basically do what the onload function already does
2b. Except we also have to clear the canvas
3. Add an HTML button or the step button
4. Define a JavaScript function that is called whenever the button is clicked
4a. Change the x-position of the center of the particle by 1

Version 4
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There is a world with 1 randomly-placed, randomly-sized, stationary particle.
1. Get random numbers for center_x, center_y, and radius
2. Make sure the circle is inside the walls

Version 3
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There is a world with 1 stationary particle.
1. Make canvas bigger
2. Create a JavaScript file
3. Link it to the HTML file
4. Add JS  code to draw a circle on the canvas

Version 2
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1. Create an HTML canvas element
2. Style it using CSS (give it a border and a background color)

Version 1
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1. Add our name
2. Add a description

Version 0
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1. Download the template
2. Upload the template to our public_html
3. Open the template in our web browser