The Shrinking Puddle
On Monday morning, a class measures a puddle on the playground and finds it is 30 cm across. They measure it again at lunch and find it is 22 cm across. By the end of the school day, it is 15 cm across. The weather is sunny and breezy, and the temperature rises from 12°C in the morning to 20°C by afternoon. The next day it rains, and a new puddle forms in the same spot. A student asks, "Where does the water go when the puddle shrinks? Does it disappear?" Another student says, "Maybe it soaks into the ground." The class wants to figure out the best explanation using evidence.
1. Use the evidence to explain what most likely happened to the puddle water. Consider both the shrinking measurements and the weather conditions.
Read the worked answer
The puddle shrank steadily (30 cm → 22 cm → 15 cm) during a sunny, breezy day as the temperature rose. This pattern fits evaporation: heat from the sun and warm air gives water molecules energy to change from liquid to gas (water vapor) and rise into the air. The breeze helps by carrying vapor away so more can evaporate.
Soaking into the ground is possible too, especially on playground soil, but the steady shrinking on a warm, sunny, breezy day strongly supports evaporation as a major cause. The water does not disappear; it becomes water vapor in the air. The next day's rain shows water can return as precipitation, completing the cycle.
2. Design a simple test the class could do to find out whether soaking into the ground also happened. Describe the test and what result would support each explanation.
Read the worked answer
One test: place an identical shallow container of water (same amount) on the playground surface and another on a solid tray that water cannot pass through, both in the same sunny spot. Measure both over the day.
- If the container on the ground loses more water than the tray container, soaking into the ground is also happening.
- If both lose about the same amount, evaporation explains most of the loss and soaking is minor.
Another test: check the ground under the puddle spot for dampness. If the soil is wet below the surface after the puddle shrinks, water soaked in. If it is dry, evaporation is the main cause. The key is comparing conditions so only one factor changes at a time.