Using a sharp instrument (easy there), slice the very bottom off the plastic container. Next, we want to convert the cap, minus the liner, into a screen. So with either a 1/16-inch drill or a pointed tool, make as many holes as practical in the cap, then cover all the holes with a piece of stocking. Screw the cap back on the container, and seat the container upside down in the stand.

Now start preparing the filter bed. First goes the fine sand, up to the narrowest part of the neck. Follow with coarse sand, up to the top of the funnel section. Then two or more inches of gravel and the same amount of pebbles.

With the bowl under the filter, pour some dirty water on the pebbles. Try muddy water, dishwater, your bathwater (the last one could prove to be the toughest test of all). What empties into the bowl should be noticeably cleaner.

But it certainly won’t be clean enough to drink. After all, we’re just trying to demonstrate a principle, not really purify water.