Earth-Water Predictions at the Handball Wall

A Break-Time Prediction Challenge

Beside a playground handball wall, a recreation leader named Imani is preparing a science challenge for break time. Players will use a bag of labeled tiles to model Earth's water. Each tile is the same size and represents the same amount of water. A player will mix the bag, choose a tile without looking, read its label, and return it before mixing again. The tiles stay dry; the labels stand for water rather than holding real water.

The first proposed model has forty tiles. Some say Salt water and some say Fresh water. The graph below shows the proposed counts. Before building the bag, Imani asks players to check whether these counts represent Earth well. A model can have the correct total number of tiles but still give the wrong share to each kind of water. The team wants predictions based on Earth's water distribution, not on which label a player hopes to choose.

For the challenge, Imani uses rounded scientific estimates. Imagine dividing all of Earth's water into 1,000 equal units. Salt water accounts for 975 units, and fresh water accounts for 25 units. That is about 97.5% salt water and 2.5% fresh water. These estimates describe water across the planet, not just water near the playground. The team will graph these amounts before revising the tile model. In a forty-tile model, each tile would represent twenty-five of the Earth model's units.

Most of Earth's salt water is in the oceans. Fresh water includes water frozen in glaciers and ice sheets, fresh water stored underground, and water in lakes, rivers, and other places. Most fresh water is frozen in glaciers and ice sheets; much of the rest is underground. A small fresh water bar does not show how that fresh water is divided among its storage places. The challenge's two labels describe water type, not whether people can easily collect the water or drink it safely.