When lettuce is growing in the field it's kept warm by the sun and the soil, which are ideal conditions for growing. But when it's harvested it needs to be cooled down straight away to keep the lettuce fresh and looking good.
Once the farmer has harvested the lettuce, they transport it back to the farm's pack house. Giant cooling units reduce the temperature of the lettuce to between 1°C and 5°C. It stays at this temperature for the rest of its journey all the way to the supermarket.