Agar is a vegetable-based gelling agent made from seaweed which has no flavor. It begins to set quickly in the refrigerator. In fact, unlike beef gelatin, gels made with agar will begin to thicken even without refrigeration. Agar comes in flakes and bars, but don't choose these if you can get the granular form; it's much easier to measure and dissolve the powdered agar. One tablespoon of powdered agar gells 3 cups of liquid.
Note from BL: Since Lady E kindly sent this information, I wanted to add that this is not the same as kosher gelatin which can sometimes be purchased in stores. Kosher gelatin is often made from beef or fish bones, etc. whereas non-kosher gelatin may be made from pork as well. Some people also have success using fruit based pectins for jelling as well.
Put the agar in your cold liquid in a pot; once it has started to swell or dissolve, heat it and stir it until it boils, and simmer about 15 minutes.
Note: It won't clump up, and it dissolves completely in whatever liquid you use. The only exception is fresh pineapple or papaya juice; the powerful enzymes in these fresh fruits will interrupt the gelling action. So if you want to use either of these, they must be cooked or canned first.
Once the liquid begins to thicken, you can add solid ingredients to it, whip it or layer it. Poured into a square pan, it will set firmly enough to be cut into crystal cubes after a few hours. Three cups of juice plus some fruit makes three individual bowls.