A vertical farm is a building full of plants. The plants do not grow in fields. They grow on shelves, one above another, in rooms with no windows. Instead of sunlight, they get light from pink and blue lamps. Instead of soil, their roots sit in water that carries all the food they need.

This way of farming saves a lot of water. The same water goes round and round in pipes, so a vertical farm can use about ninety percent less water than a normal field. It also needs very little land. A farm inside an old factory can produce as much salad as many hectares outside.

There is no bad weather indoors. Farmers do not lose a crop to rain, frost or insects, and they can harvest every week of the year. Because the farm is close to the city, the leaves reach the shop a few hours after cutting, so they stay fresh much longer.

However, the lamps use a lot of electricity, and electricity costs money. This is why vertical farms grow small green plants like lettuce, herbs and baby spinach. These plants are light, they grow in three weeks and people pay well for them. Wheat and rice need much more light and much more space, so nobody grows them this way.

So a vertical farm will not feed the whole world. It is a useful helper, not a replacement. It brings fresh salad to big cities and to cold places where little grows, while ordinary fields keep giving us bread, rice and oil.