Cleaner air, litres of water and a filter

We use water all day, so it is easy to forget how much. To find out, multiply. If a shower uses about 10 litres every minute, a 5-minute shower uses 5 × 10 = 50 litres. If a flush uses about 6 litres, four flushes use 4 × 6 = 24 litres. The numbers here are rounded, so we say “about”. Add up every row to get one person’s day, then multiply by the people in a house to get the household total. In some places each person has only about 40 litres a day for everything, so every litre counts. Saving water is a good idea even when we have plenty, because clean water takes work to bring, and the Nile gives Egypt almost all of its water. Small habits add up: a shorter shower, a tap turned off while brushing, a leak fixed.