Probably the commonest of all accidents are cuts and scratches. So common is it for us to "bark" our knuckles, or our shins, or scratch ourselves on nails and splinters and drive pins into ourselves, or let our pocket knives slip and cut our fingers, that, if the human skin had not the most wonderful power of repairing itself,--not merely closing up the cut or the scratch, but making the place "as good as new,"--we should be seamed and lined all over our hands, arms, faces, and limbs like a city map, or scarred and pitted like a tattooed man, before we were fifteen years old. But of course, as you know, the vast majority of cuts and scratches and tears heal perfectly. They hurt when they happen; and they burn, or smart, for a few hours, or hurt, if bumped, for a few days afterward; but they heal soon and are forgotten. On the other hand, some cuts and scratches will fester and throb and turn to "matter" (_pus_) and even give you fever and headache and blood poisoning. What makes the difference? It is never the size, or depth, of the scratch or cut itself, but simply _the dirt that gets into it afterward_. If a cut, or scratch, no matter how deep or ragged, be made with a clean knife-blade or sliver and kept clean afterward, it will never "matter" (_suppurate_) or cause blood poisoning. So if you know how to keep dirt out of cuts and scratches, you know how to prevent ninety-nine per cent of all the dangers and damage that may come from this sort of accident. Not more than one cut or scratch in a thousand is deep enough to go down to an artery, so as to cause dangerous bleeding, or to injure an important nerve trunk. So, though no one would by any means advise you to be reckless about getting cut and scratched, yet it is better and safer to run some risk of cuts and scratches in healthy play when young, and learn how to keep them clean, than to grow up pale and flabby-muscled and cowardly. How to Prevent Infection in Wounds. It is not just dirt that is dangerous,--although dirt of any sort is a bad thing to get into wounds and should be kept out in every possible way,--but dirt that contains those little vegetable bacteria that we call germs. The dirt most likely to contain these germs--called pus germs, because they cause pus, or "matter" in a wound--is dirt containing decaying animal or vegetable substances (particularly horse manure, which may contain the tetanus, or lock-jaw germ) and the discharges from wounds, or anything that has come near decayed meat or unhealthy gums or noses or teeth. This is why a cut or scratch made by a knife that has been used for cutting meat, or by a dirty finger-nail, or by the claw of a cat, or by the tooth of a rat, is often likely to fester and "run." Animals like rats and dogs and cats often feed upon badly decayed meat; and hence their teeth, or claws, are quite likely to be smeared with the germs that cause decay, and these will make trouble if they get into a wound.