Symbiosis
Symbiosis is when two living things help each other by living close together.
Think of it this way
A sourdough starter lives in a jar on the counter. Yeast and bacteria share that jar and help each other survive. The yeast breaks down sugar and makes gas that lifts the dough. The bacteria eat what the yeast leaves behind and make acid that keeps harmful mold away. Living side by side, each one gives the other something it needs.
Explaining symbiosis by grade level
A goldfish breathes out a gas. A water plant uses that gas to grow. The plant makes clean water for the fish. Both the fish and the plant do better together than alone.