I'll hold down the fort. He walks to the exit and turns off the lights, but instead of going out, he slides into the background. The projector comes on. The water hose expands to fill the screen, showing the passage through it nearly completely restricted with mineral deposits. Margo continues happily with her presentation.
Water contains minerals that sediment out and grow like stalactites inside pipes.
As a result, the pipe becomes clogged and will no longer pass water. The blood vessels in our bodies have a similar problem. Certain minerals cling to the walls of the arteries and plug them. She points to deposits that have grown into blockages. Some things accelerate this.