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.
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!