MY DEAR BOYS AND GIRLS: I have here to-day quite a variety of locks. Here are also quite a variety of keys. You will notice that there are several more keys than there are locks. Now, I suppose that we would have no very great difficulty in selecting the keys that would be most likely to turn backward and forward the bolts in these different locks. We would naturally expect that these larger keys would fit these larger locks and the smaller keys would be adapted to lock and unlock the smaller ones. [Illustration: Lock and Key.] Here is this large lock; I suppose it is very possible this large key may be suited to lock and unlock it. Yes, it just fits. You see how it turns the bolt in and out as I turn the key. Now, here is another lock; let us see if we can find a key that will fit it. This key seems about the size, but after passing it into the lock it seems to strike something that prevents it from turning, and consequently is of no service. Let us try another. That seems to work much better, and turns the bolt backward and forward. [Illustration: Lock and Key.] Here is still another lock; let us try this key with this lock. That seems to work very well. Possibly we might be able to lock and unlock this other also. Let us try it. Yes, this key fits both these locks. This key is what the locksmith calls a skeleton key. It is so made that it avoids the obstacles which are placed in the different locks to prevent them from being opened by all varieties of keys. Here is a still smaller lock. This lock has a very peculiar keyhole, and I know at once that there is no need of trying to unlock it with most of the keys which I have spread out here. I recognize it at once as what is called a "Yale lock." The key is thin, is bent in various ways, and along the edge has several notches. Let us try a couple of these keys. This one seems to fit very well to the grooves. It passes into the lock, but I cannot turn the bolt. Let us try another. Yes, this seems to be the one that was made by the locksmith to fasten and unfasten this lock. [Illustration: Yale Lock and Key.] A key then is simply something which unlocks the door or the gate, so you may open it and pass inside. Now, there are a great many kinds of keys. Sometimes a book is called a key to business. Perhaps another book is called a key to the study of medicine; another the key to the study of law. And so there may be a great many kinds of books which are called keys. When properly used or studied they open the way for a clear understanding of how to transact business, how to study medicine and how to study law. And so there are various books that are keys to the understanding of very, very many subjects. When you indicate to me the kind of difficulty that you have to overcome, it would be reasonably easy to indicate the kind of book you need in order successfully to meet that difficulty.