One of 9 short programmes made by Cornucopia for BBC Learning and broadcast in 2012 on the BBC2 Learning Zone. Features Stefan Gates finding out about real life environmental issues and the maths that helps us create a more sustainable world. Stefan visit a huge hi-tech greenhouse in Kent that grows cucumbers all year round. He meets Judy Whittaker who explains how they harvest water from the roof of the greenhouse, collect it in a reservoir and use it instead of tapwater to water their plants. In a demonstration, Stefan pours water from one container into another to explain how the same volume of water can come in lots of different shapes. Using a metre square frame he shows that for every mm of rain that falls on a square meter or roof we get a litre of water. Rob James helps him calculate how much water can be collected from the greenhouse roof that is 61,000 square metres. Stefan finds out that it depends on the rainfall but on average is 35 million litres per year. Stefan then meets a group of children from Peatmoor Primary School who talk about the water they need for their big ecogarden and show him their own rain harvesting project using the garden shed. Next Stefan helps them to calculate how much water they can collect from their main school roof using a scale plan of the school. First broadcast in the Learning Zone on BBC2 in March 2012 as part of the series Ecomaths.