Light a candle. Place a glass jar over the candle after it has burned for a minute. The flame will go out when it uses all the oxygen in the jar. Without oxygen, the flame cannot convert the hydrocarbon molecules from the candle into energy (heat and light).
Combine oxygen with iron to make rust. Put a nail in vinegar for a day. Then, put the nail in a jar of water. Come back a few days later to see the rust on the nail. Like the candle, this change will loosen electrons and generate heat.
Combine oxygen with iron in food to make rust. Cut an apple and leave a piece on the table. In a few minutes, the apple will turn brown. That is rust!
People and animals breathe oxygen and exhale carbon dioxide. Plants take carbon dioxide and produce oxygen. Place a glass jar over a plant and wait a few days. Cut a hole in the bottom of a large, plastic bottle. Light two candles. Remove the glass jar from the plant and place it over one of the candles. Place the plastic bottle over the other candle and breathe into the top. The candle inside the plant jar will burn better because it is receiving more oxygen.