Most bodily functions require water in order to work effectively, keeping us healthy and preventing illness. A large proportion of our body is made up of water but this is lost during the day, so it is vital that we replace the lost water to prevent dehydration. Water can be found in many places but not all of it is safe to drink. You should only drink bottled water and tap water. Rain, lake and seawater is not safe to drink because it has not been purified and may contain parasites and bacteria that can cause illness.