Let's start with a fairly simple object, whose intended use is well known: the hamster ball. Mine is labeled as being for hamsters, gerbils, and other furry friends. Unfortunately, my furry friends are of the canine variety and don't fit inside a seven inch sphere, making the intended use of this object particularly unlikely.

But just to be on the safe side, let's make that intended use completely impossible. Specifically, this begins by plugging the air holes so any furry friends that do wind up inside will be unable to breathe, let alone exercise. I did this with a hot glue gun; in hindsight, caulk probably would've been a better choice as the glue tends to flow right through the holes and not actually make any kind of seal. (Seals also don't fit inside the ball, though some at least would likely have no trouble balancing it.)

The hot glue having done it's job, we're left with a clear plastic container with a couple interesting properties. First, it's equipped with a big enough resealable hole to admit an eighteen-ounce peanut butter jar. Second, it's watertight - except for the area right around where it closes. Lastly, because it's spherical (except for deformations of the surface resulting from bumps of hot glue), it's particularly easy to make tumble. Pairing it with the aforementioned peanut butter jar, we have two containers, one of which goes inside the other with plenty of room to spare. Now let's turn to something to go in those containers. Which means, it's recipe time! Here's what I did:

Combine two thirds of a cup of whipping cream and one third of a cup of whole milk in a saucepan over low heat. Add one third of a cup of sugar and enough vanilla extract to produce a noticeable amount of vanilla flavor in the cream. ("Vanilla" is often used to mean "ordinary", but only because people are used to too little of the flavor being present. In quantity, it's a very sensual aroma.) I used somewhere around a teaspoon or perhaps a bit more. Stirring periodically, heat the milk and cream. Two things need to happen here: one, you need to get the sugar dissolved; two, you need to scald the dairy products. Pour the result into an eighteen-ounce thin-walled plastic jar of the sort that stored peanut butter in a previous life. Seal it and store it in the refrigerator for at least six hours. Overnight would probably be best - at least according to Alton Brown, who halfway stole this recipe from.

When the liquid has been in the fridge for a while, take your hamster ball. Look for the rock salt, fail to find it, and add half a cup of coarse kosher salt to the ball instead. Then add the peanut butter jar and fill it the rest of the way with ice. Make sure the peanut butter jar is sealed tightly! Now, your waterproofing of the hamster ball has one weak spot - the opening. Duct tape around the edges of that to avoid leaks.

Play with your ball any way you like to keep it moving. Personally, I recommend a game I call Lincoln-Style Solo Soccer, which consists of kicking it back and forth between one bare foot and another, and just sort of stirring it around with your feet for a bit. Warning: Actually doing this barefoot may cause discomfort in those of you unused to being in that state. Whatever you choose to do with the ball, keep it in motion for at least ten minutes. If it isn't making a noise like a hotel ice dispenser and scaring your oversize hamsters to the far side of the house, you aren't churning properly.

Then, break the seal by removing the duct tape. Open the ball up and pour off any liquid water. Try to refill with ice, realize all four of your ice trays went into the first loading and are still liquid from refilling, shrug, don't add anything, and play with the ball for at least another ten minutes. Break the seal again (damn flimsy animal crackers!) and remove the peanut butter jar. That goes into the freezer for an hour - which by the way is code for "probably should be overnight but you're impatient." Wash out the hamster ball and set it to dry. Realize from the spot or two of water where you were playing that you'll likely have to repair the glue seals before its next use.

When the hour is up, serve the delicious if suboptimally textured (for exactly why, see preceding recipe) ice cream contained in your peanut butter jar. Then, post an SF0 praxis about the experience.

Credit where credit is due: this idea was taken entirely from a certain Instructable on the subject. The recipe was not included, but is adapted from one on the show linked above.