You will need A glass Tap water Red or blue food coloring. Here's what you do. Fill the glass with tap water. Add two or three drops of food coloring. Notice how it spreads through the water.
Pollution spreads through water just like the food coloring does.
Wash the piece of celery. Leave the leaves on. Put the celery in the glass filled with colored water. When the time has passed, take the celery stalk out of the water. Do this several times.