A terrarium provides an opportunity to explore the water cycle.

This investigation can take several weeks and forms a great project.

Take two identical large, transparent glass jars with lids. Put a small handful of gravel in the bottom of each one. Mix a handful of sand into some soil and place a layer of this soil-sand mix into each jar so the jar is almost half full. Add equal amounts of water to each jar, ensuring there is enough water to make the soil moist, but that it is not waterlogged.

Choose one of the jars to plant seeds in. Make sure you choose quick-growing seeds such as herbs and plant them according to the instructions on the packet.

Put the lids on both jars and stand them in a sunny place, making sure they are not in direct sunlight.

Make a chart to record your observations each day. Make sure you note whether there is any water on the glass, whether it looks misty inside the jar, whether the soil is moist or not, and when the plants start to grow.

You also need to record the weather conditions outside the jar. You may need to add more water occasionally - if so, make sure you add the same amount to each jar.

Over the weeks you will observe the water cycle, as water in the soil evaporates. The mist you sometimes see is water vapour, and the water on the glass is condensation.

As the plants grow they cover the soil, and less water evaporates. This means the conditions in this jar are less extreme than the conditions in the jar with no vegetation.

For more information, follow-on activities and curriculum links try www.planet-science.com

Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn