It's la rentrée, and our local paper came up with an article about one of those problems that appear even in the best of homes: head lice.
Very contagious (a bit like verrucas in swimming pools), but easy enough to cure if you do it right. The article commented that ordinary shampoos are no use - you need an insecticide shampoo - and that it is more effective to use a cream and leave it for a long period.
However, this being France, they also mentioned a folk remedy that they claim works every time: mayonnaise
You slather the child's head in mayonnaise, then let him/her sleep wearing a shower cap and wash it off in the morning.