Science activity
Water Cycle in a Bag
Watch a tiny water cycle happen inside a bag.
Materials and setup
- zip-top plastic bag
- water
- blue food coloring (optional)
- tape
- sunny window
How this changes by age
Pre-K (ages 3–4)
Watch a tiny water cycle happen inside a bag.
Steps
- Pour a quarter cup of water into a zip-top bag. Add one drop of blue food coloring.
- Seal the bag and tape it to a sunny window.
- Check it every hour today. Tell what you see.
- Notice: tiny drops appear on the inside! Where did they come from?
- Tell a grown-up one thing that surprised you.
Learning objectives
- Observe evaporation and condensation
- Build curiosity about water
Kindergarten (ages 5–6)
Make a mini water cycle and learn its three parts.
Steps
- Set up a bag of water on a sunny window (with optional blue coloring).
- Draw the bag at the start. Draw it again every 2 hours for 6 hours.
- Notice: water disappears from the puddle and reappears as drops on top. Then drops fall back down — that's rain!
- Learn three words: EVAPORATION (water becomes invisible), CONDENSATION (water drops form), PRECIPITATION (drops fall).
- Say the words out loud and point to where each one happens in your bag.
- Tell a grown-up one thing that surprised you.
Learning objectives
- Identify the three stages of the water cycle
- Use scientific vocabulary
- Make repeated observations
Early elementary (ages 6–8)
Compare water cycles under different conditions.
Steps
- Set up THREE bags: one in direct sun, one in shade, one in the refrigerator. Same amount of water in each.
- Predict: which bag will form drops fastest? Why?
- Check every hour for 6 hours. Record observations in a chart.
- Which bag formed the most condensation? Which formed the least? Why does temperature matter?
- Connect to real life: clouds form when warm wet air rises and cools. Where does rain come from?
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Run a controlled experiment
- Compare conditions
- Connect lab observations to weather
Upper elementary (ages 8–10)
Quantify the water cycle and explain phase changes.
Steps
- Measure exactly 50 mL of water into a bag. Tape to a sunny window.
- Photograph the bag every hour for 8 hours. Estimate how much water has evaporated by visual volume.
- Calculate the rate of evaporation in mL/hour. Now predict: how long until ALL the water has cycled?
- Research the molecular explanation: water molecules absorb energy, break their hydrogen bonds, and become vapor. Cool surfaces give back that energy and the molecules re-bond as liquid.
- Write a 1-paragraph explanation of how this same cycle moves water through Earth's atmosphere — including evaporation from oceans, transpiration from plants, and precipitation as rain or snow.
- Look up Earth's annual rainfall total. How does that compare to your bag's tiny cycle?
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Measure evaporation quantitatively
- Connect molecular behavior to phase changes
- Relate lab to global water cycle
Safety and evidence note
Read the full activity before beginning. An adult should supervise tools, heat, food, outdoor work, movement, and experiments as appropriate. Completion records that the activity was done; the child’s explanation, work sample, photo, or demonstration is stronger evidence of learning than a completion check alone.
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