I’m wondering, is it OK, hypothetically, for a child to lick the sole of her shoe? What about, just for argument’s sake, eating a cigarette end in a park? And where, as a general rule, do we stand on licking sticks? In other words, sometimes I feel there isn’t enough hand sanitiser in the world to keep a small child clean. All these questions go through my mind as I prepare to speak with microbiologists Marie-Claire Arrieta and Brett Finlay, whose book for parents, Let Them Eat Dirt, has turned my world on its head. Full blog