What you need:
- plastic cup
- food coloring (green for this one, but you can use any color)
- knife (an adult should use this)
Plants use their roots to "drink" water from soil. Even after a flower has been cut and no longer has roots, it uses its stem to suck up water for the flower and leaves. As water evaporates from the flowers and leaves, more water is sucked up through the stem, almost like a straw. The colored water doesn't hurt the flower, but allows us to see how the water moves through the plant.
You can read more about the experiment and ideas for further experimentation on Steve Spangler's website.