If you are planning to introduce the phrase “let the cat out of the bag” into a conversation, you can give your words a weightier punch by having a bag with a cat in it, ready to be released at the right moment. This is a variant on the argument from demonstration, and when we are looking at methods of adding heft to what we say, it can be very effective.

The technique is simple. First, obtain a cat. The standard domestic moggy will suffice, and you can pick one up from an animal shelter or pet shop. I do not recommend abducting one while it is dozing in the sunlight on the windowsill of a happy family home, for that would be very wrong of you, and though you probably would not be sent to prison, a black stain would be seared upon your immortal soul for the crime of causing misery to blameless citizens, leaving bereft infants sobbing into their pillows as they spend fitful nights lamenting the loss of Tiddles. Make a note of that, in case you are tempted to cut corners and go out marauding, on the lookout for the first cat to cross your path.

Choosing a bag is slightly more problematic, but only if you are dim. Clearly the bag must be big enough for the cat to fit inside it, and it must have some form of clasp or seal. If you have not grasped that much, go and get your brain hardened up at a cranial integument-stiffening facility and come back when you are less of a fool. The rest of you will have gathered that, as long as it meets the above provisos, pretty much any bag will do. It is unwise to spend excessively on some kind of designer handbag, a choice you might make if you are an oligarch or a plutocrat. Even if you have the riches of Croesus, there are much more constructive uses for your money, such as donating goodly sums to a charitable fund for out of print pamphleteers. Incidentally, the act of making such a donation is guaranteed to remove from your immortal soul any black stains or other besmirchments with which it has been seared or smeared, so listen up! I am not suggesting that by handing your money over to an impecunious scribbler you are thereby excused snatching a cherished family pet from its windowsill slumber, so don’t go thinking you can weigh the evil act against the good. Anyway, just get the damned bag and put the lawfully-obtained cat into it. Simply pick up the cat, put it in the bag, and use the fastening mechanism, whatsoever it may be, to ensure the cat stays put until you are ready.

Let us leap forward in time to the exciting moment in the conversation that all this is leading up to. You are sitting in an armchair. The bag with the cat in it is on your lap. To add a vivid if unnecessary detail, let us picture your interlocutor, looming over you, standing at the fireplace, stabbing the air with his pipe as he speaks. “Blah blah blah,” he says. His words are unimportant in this context, for all our attention is rightly upon you – you who have learned that it pays to increase your word power. “Blah blah blah,” he continues, seemingly unstoppable, until he pauses to take a puff of his pipe, aglow with expensive Montenegrin tobacco obtainable only through a specialist supplier in the most exclusive quarter of your city. You take the opportunity of his brief silence to say, “Ha! That’s let the cat out of the bag!” and as you crow the words, you unfasten the bag and let the cat out of it. Quod erat demonstrandum.

This ploy will work on a handful of occasions, after which it begins to lose its effectiveness. Thereafter, both the bag and the cat will need to be given new purposes in your life. You can use the bag to haul potatoes back from market, or to place as a hood over the head of any hostage you see fit to take, if you are that way inclined. As for the cat, it will be happy with regular bowls of milk and a supply of maimed but still-living field mice or small birds.