A circle increases its radius by 5 every tick. Hitting a key reduces the radius by 50%.
A square moves along a horizontal line. Every time you move the mouse, it moves right 1 pixel. When you hit a key it moves left 4 pixels.
A small solid circle randomly moves between x=20, x=60, and x=100 on a fixed horizontal line. (You pick the $y$ coordinate.)
Make the graph of $y=x(200-x)/50$. See below.
Method for graphing $y=f(x)$ using build-image: color points based
on $c(x,y) = y-f(x)$. When $c(x,y)=0$, the point $(x,y)$ is exactly on the graph. Depending on how thick you want the line, try $c(x,y)<5$ to make black or gray.
(define (c x y)
(- y (* 1/2 x)))
(define (color-f x y)
(gray (limit (* 50 (c x y)))))
(define (gray n)
(make-color n n n))
(define (limit n) n) ; FIX: you should fix this one yourself