Watch the video and notice that the water pours down into a swallow hole and comes back out as a spring. This is because the limestone is made of blocks and as the water passes through the cracks it dissolves them and makes them bigger.

You can model this by using sugar cubes. Place some sugar cubes on a board. Four layers of cubes will work well. This is your limestone rock.

Wedge up the board at one side so the board has a gentle slope. Now put a sheet of thin clear plastic over the uppermost third of the 'limestone'. Pour water on to this plastic from a jug. A slow, even flow is needed to make a small 'river'.

The water will flow off the plastic and down the gaps between the sugar cubes. the water will dissolve the eges of the cubes away and come out as a spring at the bottom.

Watch how the 'limestone' and 'spring' change over time.