Science is never easy but it can certainly be fun. The "Celery Science" experiment is a classic demonstration in the primary classroom. It clearly demonstrates how water moves though plants and teaches the students what a "control" is in any experiment.
Get into character. Before I teach any science experiment I leave the room and re-enter as "Dr. Science." I put on a lab coat and some glasses to get into character. The kids think it's hilarious.
Lab coats are not diffucult to acquire. Walk into a hospital or doctor's office and they will probably give you one if tell them you are a teacher. Show credentials if necessary. Or, try a medical uniform store.
Explain to the students that just as people have veins to make our blood flow though our bodies, plants have veins that water flows through. The plants "sucks up the water" from the dirt and the water flows through the plant.
Ask the students, "How can we tell the water is going through the plant?" They will probably not come up a viable way so ask them, "What if we put colored water in the cup with a piece of celery sticking out? Would we see the water flow through the celery?"
Give each student a cup and a piece of celery. I usually let them choose the color of their water.
Place a piece of celery in a cup with regular (clear) water and explain to the kids that this is the "control." We need to see what happens to celery when we just put it in clear water. Let's make sure that the leaves do not turn different colors on their own."
Expect the kids to spill if you are letting them pour and mix their own cups of water.
Place each child's cup on a 3x5 card with their name on it. Do this for the "control" cup also.
Observe the celery the next day. If the celery was in red water the tips of the leaves should be turning red. You can the cut into the celery and see that the veins are filled with red water.