1) Take Celery or Napa Cabbage (largest you can find) and slice a stalk halfway up the middle.
2) Place two clear containers close together. Add a few drops of food colouring in two different colours to the jars.
3) Add water to the jars to fill to about 1.5 inches deep.
4) Wait about 8 hours and observe the difference!
Plants use little tubes to transport water from where they have it (the ground, usually) to where they need it (in their leaves) where it's "breathed out" through little pores called stomata.
These tubes are called 'Xylem'. When we place the bottom of the cut stem in dyed water, the leaf continues to breathe out water through the stomata, and that acts like sucking pressure in the straw-like tubes of the xylem, so the dyed water is drawn up through the leaf. On the way, it colours the xylem tubes and you can see how they network through the leaf.
There is another set of tubes called phloem that carry sugars and other food from the leaves (where they are made from the energy of sunlight) to all other parts of the plant that need it.