Find a smallish glass or jar and fill it 1/4 with water. Then pour some vegetable oil on top of that. You'll want at least a couple inches worth of oil so that the bubbles have a place to go. The oil and water will separate (as oil and water do). Then add a few drops of whatever color food coloring you want your lava to be. We did one purple and one blue.
This part was pretty cool by itself. We watched the drops of dye fall through the oil and come to rest on the top of the water. Then, after a few seconds, the dye would break through and permeate the water. Cool!
Finally, once the water is colored, add 1/3-1/2 of an alka seltzer tablet and watch the bubbles go. Once they die down, add more alka seltzer. We went through 7 tabs and could have done more but I told Joshua that we should give the oil and water a chance to re-separate and then he got interested in something else. This was really super cool!