Place a Pyrex funnel mouth-down in a saucepan full of water so that the narrow tube of the funnel protrudes above the water. Rest a part of the funnel on a nail or a coin so that water can get under it. Place the pan on a stove and watch the water as it begins to boil. Where do the bubbles form first? Why? As the bubbles rise, they expand rapidly and push water ahead of them. The funnel confines the water, which is forced up the tube and driven out at the top. Now do you know how a geyser and a coffee percolator work?