Break the Alka-Seltzer tablet(s) into quarters or halves. We like to “go big” in my classroom so we go with half a tablet at once. Then just watch and be dazzled! The food coloring will start “erupting” all sorts of grooviness in your bottle! Kids have so much fun watching the the food coloring as it bubbles and moves. As soon as the Alka-Seltzer has dissolved, you can add more or more for as long as you’d like. Image.