I just had a smallish ball of yarn handy. I played around and knit for a bit, and then decided to make a knitted ball.
There are a lot of patterns online for this, but I used a simple short rows technique from a book from the Seventies, A New Look at Knitting, by Elyse and Mike Sommer (which is a excellent book to start with if you want to knit foofy shapes). Since Google didn’t help me find a pattern that explained this method of knitting a ball very well, I’ll do it here.
In knitting, you make a short row by turning the knitting before the end of the row, leaving some stitches unknit for the time being. You may have to bring the yarn between the needles (depending on stitch pattern) before you start knitting again. There are techniques for wrapping or tightening the yarn to avoid making a small hole at each mid-row turn, if you wish, but I’d skip that at first (actually I only just learned about those today).
So, if you start with 20 stitches on the needle, knit 18, turn, knit 18, you’ve created one more row on the starting edge, but not on the other edge. (I’m not saying left or right edge here, because I am left-handed and knit “backwards”, from left to right.) The needle holding the stitches is now not quite parallel to the bottom edge. You could knit straight at this point, making a strip that has a slight kink in it, or you could knit another short row, shorter than the first one, and kink it even further. Kink it far enough, and the strip will bend around into a circle.
To make a circle, you keep shortening the rows by two stitches at a time until you run out of stitches, then knit all the way to the end of the row and back. This makes one wedge-shaped segment (a slightly distorted triangle, really). Repeat 11 times to make a total of 12 segments, cast off and join the last row to the first row, and you have a circle.
To be more specific in knitting language, here is a pattern (20 stitches is enough to give you the idea without being tediously large):
Simple short rows garter stitch circle
Cast on 20 st, knit one row.
*Begin short rows for segment:
K 18, turn, K 18.
K 16, turn, K 16.
K 14, turn, K 14.
K 12, turn, K 12.
K 10, turn, K 10.
K 8, turn, K 8.
K 6, turn, K 6.
K 4, turn, K 4.
K 2, turn, K 2.
Knit down and back along entire row: K 20, turn, K 20.
Repeat from * to make a total of 12 segments. Cast off and join first row to last, and sew hole in center closed.
To make a ball, you work short rows at both sides of the knitting, kinking in both sides at the same time. A little mind-blowing, but it works:
Simple short rows knitted ball in garter stitch
Cast on 20 st, knit one row.
*Begin short rows for segment:
K 18, turn, K 16.
K 14, turn, K 12.
K 10, turn, K 8.
K 8, turn, K 6.
K 4, turn, K 2.
Knit down and back along entire row: K 20, turn, K 20.
Repeat from * to make a total of 12 segments. Cast off, insert stuffing, join first row to last, and sew holes at top and bottom closed.