Take a pan full of water and invert pyrex funnel and rest the wide mouth of the funnel on coins. now start heating this entire setup with the help of splinter. first the coin gets heated up because it is the impurity in water.and bubbles comes from the place where impurity(coin) is present.slowly the water surrounding the coin gets heated up. as the upper water exerts pressure on the lower part of water, the boiling point of the lower water increases and the lower water is heated to great extent.the bubbles tries to come up through narrow end of the funnel. but the narrowness of the funnel exerts more pressure on the deeper parts of the water resulting in increase of boiling point. when pressure increases on the deeper portion, the vapour present there tries to escape into atmosphere. this vapour escapes with great force and carries the deeper water with it. the hot water comes out of the narrow end like a fountain.
This is the principle used in working of geyser.