Teaching the Water Cycle with a Zip-Top Bag and Sunlight

A clear zip-lock bag, a permanent marker, water and a window that catches the afternoon sun are all you need to turn evaporation, condensation and precipitation into something children can actually watch happen. The setup costs less than a takeaway coffee in Brisbane and travels well between classroom, homeschool table and kitchen bench, which is handy when rain interrupts an outdoor lesson or you simply need a science demonstration that fits on a windowsill.

For Australian teachers and parents, the activity lines up neatly with the science strands of the Australian Curriculum, where students are expected to describe how water cycles through the environment. It works just as well for early years learners exploring weather as it does for upper primary students investigating how heat energy drives changes of state, and it doubles as a low-prep option for relief teachers filling in for a colleague on a Wednesday arvo.

Gathering the Materials

A standard sandwich-sized resealable bag does most of the heavy lifting. Add a few drops of blue food colouring to roughly 100 mL of tap water so the liquid inside is easy to see against the plastic, and seal the bag almost completely, leaving a small pocket of air trapped near the top to mimic the atmosphere. A whiteboard marker writes clearly on most freezer bags, though a chinagraph pencil works even better because it does not smear when the condensation builds up.

For the window side of the experiment, choose a north-facing pane if you are in the southern parts of the country, or a north-east facing pane if your school is further north, so the bag receives direct sun for at least an hour. Sticky tape, Blu-Tack or a strip of masking tape will hold the plastic flat against the glass, and a sheet of dark cardboard behind the bag helps the water and vapour stand out instead of disappearing into bright daylight.

Drawing the Diagram on the Bag

Before the bag goes near the window, students can sketch the water cycle directly onto the plastic with their marker. A simple sun in one corner, a cloud at the top, a wavy line for the ocean at the bottom and arrows showing evaporation, condensation and precipitation gives younger learners a visual scaffold. Older students can add labels for transpiration from leaves or include groundwater flow beneath the sea line.

The act of drawing doubles as a formative check. Teachers can wander the room and see who understands that evaporation rises, condensation forms droplets and precipitation falls back down, without needing a single worksheet. By the time the bag is taped up, each child has already produced a personal diagram they can refer back to during the rest of the lesson.

Watching the Bag React to Sunlight

Within ten minutes on a sunny Perth or Adelaide windowsill, the bag begins to fog as water vapour rises and condenses on the cooler plastic. Droplets form along the upper edge first, then grow heavy enough to run back down in tiny streams, mimicking rainfall on the inside of a tiny atmosphere. Students can time how long it takes for the first droplets to appear and compare results with bags placed in shaded spots.

The contrast between sunny and shaded bags is where the real learning happens. Children quickly notice that the bag in direct sun cycles faster, while the shaded bag stays mostly clear. This leads naturally into a conversation about how heat energy from the sun powers the whole system, and why inland towns such as Alice Springs or Mildura experience such dramatic evaporation rates compared with coastal Hobart.

Linking the Demo to Local Weather

Once the bag is cycling, the activity links easily to real data students recognise from the nightly news. Pull up the current radar on the Bureau of Meteorology website, look at the synoptic chart for the day and ask children whether their bag model matches what is happening over Sydney, Melbourne or wherever their town sits on the map. Students often notice that inland rain events build differently from coastal drizzle, which mirrors how condensation behaves inside the plastic.

This is also a chance to discuss why water security matters in Australia. References to the Murray-Darling Basin, the Great Artesian Basin or the work of WaterAid Australia can sit comfortably alongside a simple bag on a window, helping children connect a kitchen-table experiment to the big conversations happening in the country about drought, irrigation and river health.

Exploring Light, Reflection and the Sun's Energy

After the cycle is running smoothly, you can branch into related investigations that explore how light behaves on water surfaces. Reflections in puddles, the glare off a dam at sundown and the shimmering effect of heat haze are all moments where symmetry meets physics, and a quick look at a light cube symmetry activity opens up useful follow-on ideas for the science table.

Light cube sessions also let children track how shadows shift through the day, reinforcing the same sun-driven energy story that powers the bag. Pairing the two activities in a single afternoon gives students a fuller picture of how light, water and temperature shape the everyday environment they walk through on the way to the bubbler.

Adapting the Activity for Different Climates

Teachers in tropical Cairns or Darwin may find that condensation appears almost instantly, while colleagues in Hobart on a cold winter morning may need to give the bag an extra hour of sun before droplets form. The activity scales nicely: a bigger freezer bag, a splash more water and a stronger food dye will help students in bright, humid regions see the cycle clearly, while those in cooler climates can hold the bag over a warm radiator for a few minutes to kick-start evaporation.

Seasonal variations also offer a teaching moment. Running the same experiment at the start of summer and again in the depths of winter gives children concrete evidence of how temperature changes the speed of the cycle, and ties neatly into discussions of bushfire season, storm build-up and the long dry spells that shape life across the country.

Crossing Curriculum Boundaries

The bag works as a launching pad for literacy as well as science. Students can write a procedural recount of the experiment, sequencing the steps they followed and the observations they recorded, which hits writing targets without feeling like a separate lesson. In maths, they can measure the depth of water at the start and end of the day, chart droplet counts at five-minute intervals or calculate how much water was lost through evaporation.

For health and outdoor education, the activity also opens a short conversation about sun safety. Tying it back to the Slip, Slop, Slap message and explaining that the same UV that drives the cycle can also burn skin in a Melbourne summer keeps the science honest and grounded in everyday experience. By the time the bag comes down from the window, pupils have practised reading, writing, measuring and questioning, all while watching one of the planet's most important systems run on a small patch of plastic.