Parents make sure their baby is clean. Parents do their best to keep dirt away from babies. However, a scientist says dirt makes babies stronger. Dr Jack Gilbert did research into dirt, germs and children. His research is in a book. He said: "It's fine to wash their hands….but if a dog licks their face, that's not a bad thing."

Dr Gilbert said babies should get dirty. Dirt helps babies to fight illnesses. Children get allergies or asthma because parents clean everything too much. This makes children's bodies weaker. Gilbert likes the "five-second rule". This is when you eat something that fell on the floor for fewer than five seconds.