I always find myself needing more juggling balls- a friend has "borrowed" mine and kicked it flat playing hackeysack, I lose one in the airport, I learn how to juggle more balls...
Though you can make do and juggle nearly anything, I really like to juggle with nice, crocheted balls. And when you make your own, you can make them exactly the fullness, dimensions, and weight you're looking for.
This tutorial also works for hackeysacks.
Step 1: Materials
You need some string, a crochet hook, and something to fill the hackeysack with (more on that in the "filling" step).
Step 2: Starting
Tie a slip knot in the end of your string. Insert the crochet hook through and pull some string through- your first stitch. Make a couple of stitches (just 2 or 3 total) in a line, then loop it around and join the circle.
Step 3: The Bottom
Work your way around the circle. Turn it into an expanding hexagon by double-stitching every 1/6 of the way around.
Keep expanding it flat until the minimum dimension is 2" and the maximum dimension (point to point) is 2.5".
Step 4: The Sides
Stop double-stitching and keep working your way up. Your hexagon will round out as the piece takes on three dimensions.
Keep going until you've gone up 2".
Step 5: The Top
Work the decrease in a hexagon by skipping stitches every 1/6 of the way around.
I marked my 1/6ths with pins as shown.
Stop before you've closed it off too far.
Step 6: Filling
Fill your hackeysack/juggling ball with whatever material you prefer. Beans work really well. I've used acorns and even gravel before, and they've worked fine. You can also fill with little plastic pellets. Just make sure whatever you use is bigger than the biggest hole in your crochet weave.
I used quilt batting here. It was the only thing I could find, and I don't really recommend it. It works okay for a juggling ball, though.
Step 7: Close Top
Keep working the decrease- a bit more difficult now that it's full!
Keep on stitching around and inward until it's all the way closed.
Cut the string, leaving a tail.
Pull the tail through the last loop.
Use your crochet hook to pull the tail into the ball, hiding it.
Step 8: Hooray!
Now you have a ball!
The dimensions I used are roughly that of a tennis ball. Actually, if you ever want to cover a tennis ball, one fit perfectly inside this ball. I don't know why you would do that, though. For a cat toy, maybe?