Plankton
Plankton is a mix of tiny plants and animals that drift in oceans, lakes, and ponds.
Think of it this way
A big soup pot holds water, vegetables, and tiny crumbs all floating around together. The crumbs and small bits do not swim — they just drift wherever the water moves them. Some bits are from plants (herb flakes), some are tiny animal pieces (meat crumbles). The whole floating mixture is what plankton is like in a lake or ocean.
Explaining plankton by grade level
Scoop up pond or sea water and look through a lens. You will see tiny living things floating in it. These are plankton. Fish and whales eat them, so they are a key food source in the water.