Students can bring in materials to use and share. The marbles and size restrictions help standardize the races. This activity is likely to produce splashes and spills, so it should be done in a location where this is not a problem. Start with observations of zooplankton and phytoplankton. Use pictures, photos, and samples to allow students to observe their shapes, projections, and behaviors.
Most plankton are heavier than water and tend to sink.
Ask how they might stay up in the water. Make a list of the students' observations. Encourage connections to what they know about density. Some of the students should notice that many plankton have long projections or antennae or hairs. Have them speculate on how these would affect movement through water.