Protoplasmic Streaming
Protoplasmic Streaming is the flow of fluid inside living cells that moves food and waste around.
Think of it this way
A bowl of soup has small bits of food floating in the liquid. When you stir the bowl, the liquid moves in a loop. The bits of food get swept along with the flow. Inside a cell, the fluid does this same loop on its own, moving food and waste from place to place.
Explaining protoplasmic streaming by grade level
The stuff inside a cell moves around, like a slow river. This flow brings food to all parts of the cell. It also takes waste away. When you add salt or other things to cells, the flow can speed up or slow down.