The Feeder Refill Water Model

A Water-Drop Sign at the Backyard Feeder

At the edge of a backyard, a family keeps a bird feeder near a small observation table. Birds visit the feeder, but the family does not collect or disturb the birds. Instead, they use the table to share science models with visitors. One afternoon, Rowan and Suki design a sign about the water on Earth.

Their sign uses 1,000 equal water drops to stand for all of Earth's water. The drops are divided into two groups: 970 drops stand for salt water, and 30 drops stand for fresh water. The numbers are rounded estimates for a model, not a count of drops that could be collected in the yard. The group checks that the two amounts make the whole model.

The sign also has a separate close-up model. This close-up divides the 30 fresh-water drops into 100 equal fresh-water units. In that model, 69 units represent water frozen in glaciers and ice sheets, 30 units represent groundwater, and 1 unit represents water in lakes, rivers, soil, and the atmosphere. The close-up is about where fresh water is stored; it is not a third kind of water to add to the salt-water and fresh-water bars.

Rowan wants to make a bar graph for the main sign. Suki reminds Rowan that both bars need one shared vertical scale, the same starting point, clear labels, and a title that tells what the amounts represent. They plan to test other models by checking whether the salt-water share and fresh-water share stay the same as in the 1,000-drop model.