They are, roughly speaking, as follows:-- First, stick to one excuse.

Thus if a tradesman, with whom your social relations are slight, should chance to find you toying with the coppers in his till, you may possibly explain that you are interested in Numismatics and are a Collector of Coins; and he may possibly believe you.

But if you tell him afterwards that you pitied him for being overloaded with unwieldy copper discs, and were in the act of replacing them by a silver sixpence of your own, this further explanation, so far from increasing his confidence in your motives, will (strangely enough) actually decrease it.

And if you are so unwise as to be struck by yet another brilliant idea, and tell him that the pennies were all bad pennies, which you were concealing to save him from a police prosecution for coining, the tradesman may even be so wayward as to institute a police prosecution himself.

Now this is not in any way an exaggeration of the way in which you have knocked the bottom out of any case you may ever conceivably have had in such matters as the sinking of the _Lusitania_.