How to Explain Photosynthesis to Kids

Photosynthesis sounds like a word to dread, but the idea is wonderful: plants make their own food, using light, water and a gas from the air. If a child can hold onto that one sentence, the rest can be added over several years.

The simple version

“A plant is like a little food factory. It takes in sunlight, water through its roots and a gas called carbon dioxide from the air through tiny holes in its leaves. It uses the light’s energy to turn them into sugar, which is its food. It also gives off oxygen.”

Write the recipe on paper:

Water + carbon dioxide + light energy → sugar + oxygen

Children over eight or nine can read this as an equation. Younger ones can just hear it as a recipe.

For ages 4 to 6: what do plants need?

Skip the word photosynthesis. Ask, “What does a plant need to live?” Get them to guess, then test.

Place one bean seedling in a sunny window and another in a dark cupboard for a week. Compare color and height.
Water one pot and not another.
Draw each plant before and after.

The leaves of the plant in the dark turn pale and the plant looks weak. You can say, “The leaves need light to make food.” Our lesson What does a seed need to grow? is a good starting point.

For ages 6 to 9: where does a plant get its food?

Many children think plants eat soil. This is the most common mix-up, and it is worth testing gently.

Ask: “If a plant eats soil, what would happen to the soil as the plant grew?” Weighing is a good way to find out. A famous idea in the history of science is that a growing tree gains far more mass than its soil loses. You can do something similar with a bean plant: weigh the dry soil in the pot at the start, grow a plant for a few weeks, then (an adult can help) weigh the soil again after drying. It barely changes.

So where does the plant’s body come from? Mostly from the air and water. That is a surprise worth sitting with. The lessons Where does a plant get its food? and Where does a tree’s mass come from? take this further, and the matching blog post is where does a tree’s mass come from.

For ages 9 to 12: see it, measure it

Leaf disk experiment. Punch small disks from spinach leaves, remove the air, and watch them rise in a bicarbonate solution under a lamp as the leaf makes oxygen. This takes some preparation and an adult should help with the syringe and solution. The blog post on the spinach leaf disk photosynthesis experiment gives the steps.

Covered leaf. Cover part of a leaf on a living plant with foil for several days, then compare it with the uncovered part. Do not damage the plant. The covered patch looks paler.

Pond or aquarium plants. If you have a pond plant in a clear container in sunlight, you may see tiny bubbles forming on the leaves. They are mostly oxygen.

Common mix-ups

“Plants eat soil.” Soil provides water and small amounts of minerals. The sugar comes from photosynthesis.
“Plants breathe in oxygen like us.” Plants use oxygen too, for respiration, day and night. In daylight they make more than they use.
“Plants only need water.” Light is essential.
“Plants make food at night.” Without light, they cannot.

Tying it to food

Everything you eat started with a plant, or with an animal that ate a plant. That makes sunlight the starting point of most food chains. Ask your child to trace their lunch back to the sun: a sandwich, an apple, a glass of milk. The lesson How does energy from the sun move through a whole ecosystem? gives older children a structured way to do it.

Safety

Do not eat any plant you have used in an experiment, and wash hands after handling soil. Keep plant experiments away from young children who put things in their mouths, and check that houseplants are not poisonous to children or pets before using them.

Where to go next

The photosynthesis unit is a full sequence for older children, and the plants and seeds unit suits younger learners. If a child is puzzled by fruit and flowers, is a tomato a fruit? is a fun detour.