Any cup or jar can be a planter, but what makes this special is the bottom reservior for water storage and drainage. It keeps the roots from staying wet (assuming you don't over-water it), which can lead to plant-destroying fungus, but still lets the plant drink whatever it needs. The reservior also gives it a nice weight, for stability.
These arrange well on a windowsill, and when it's time to replant it outdoors, you can simply pull it off the reservoir, then push it through the upper cylinder as it neatly drops into its new home.
Insert absorbent cotton fabric, a sponge, paper towels, or a similar material through the top hole of a new can.
Now remove the top of a second can, then its bottom. Shove it over the top of the first can so that it seats firmly.
If you like, cut and fold-in the upper edge to make it look finished, since we don't yet sell the attractive plastic lip.