Tim Key: I have five of mocha in front of me. This is a joke. At least, I think it is

When is a joke not a joke? Not sure, really. I suppose when one person thinks it's a joke and the other person thinks it's just a normal bit of life. I'm pretty sure I've just experienced this phenomenon.

It's the reason I'm sat at a table in the corner of a café in deepest Soho, with five mugs of mocha in front of me. I'm alone at the table. All these mochas are for me. One, two, three, four, five. Five mochas. Too many mochas. But I meant it as a joke.

The waitress in question is still chewing her bubble-gum and floating about the place in her American diner-style waitressing outfit and baseball cap. She sometimes looks across at me quizzically. She's trying to give me the benefit of the doubt, I think. Trying to invent ways in her own mind of this situation making sense. But she's having a hard time of it and so am I. I don't think she should have given me five mochas. She doesn't think I should have ordered them. We both think that the other is insane. I don't know whether she has a column in which to put her side of the story; fortunately I do.

Here, then, is the chain of events, which led to this 1.5 litre approx of mocha:

1. I walk into the café. The waitress takes my order. My order is "a mocha". She smiles and asks if I want food. I tell her no, I'm full, and off she goes. Minutes pass. I admire the 1950s American décor; the chrome counter stools; the rollicking jukebox. Then a man arrives with my mocha. He plonks it down. I thank him. End of Part 1.

2. After some more moments, my gum-chewing waitress reappears with a further mug of mocha and a look of surprise that I already have one. "Is that a mocha?" she asks with, to be fair, a smile. I confirm. "Oh, this is your mocha" – she points at her freight. I nod. She puts it down and goes to do some detective work. I now have two mochas and quite enjoy how it makes me feel. If there's been a screw-up in the kitchen I might get to keep both. My heartbeat quickens. End of Part 2.

3. She returns to my table, laughing. She picks up the second mocha. "This one's for him." She points to a brute. I laugh along, wave goodbye to my bonus mocha, she delivers it to the brute and then she wafts by my table one last time. We both smile about the mix-up, her corn-coloured hair patting against her rosy cheeks. "Would you like anything else?" she asks. "Yes," I say, suddenly going serious, but my eyes still sparkling like gemstones. Joke-mode. "Four more mochas, please." She writes it down – nice touch – and closes her pad. She is very deadpan as she does this and I enjoy it no end. End of Part 3.

Part 4 you know already.

She didn't use a tray, just two trips. A mocha in each fist, then back to the Classic American Diner Counter for the other two. So four mochas. Plus the original one. Five. I thanked her. I didn't know whether she was still joking. Whether she'd thrown a bit of money at the problem and bought these mochas for the sake of the joke. Props comedy. Initially I thought that must be what had happened. But she wasn't laughing. If anything she was judging. She was looking at me in an interested way. The way a naturalist scrutinises a new species. And then she moved away.

So here they are. Five mochas. I've already had three-quarters of the first one. She looks across at me. I look back. Neither of us are laughing any more. I'm drinking these. See how she deals with that as a punchline.

I tap the mochas with my pen as I try to work out my next move. As I move along I tot up the price. "£3.50, £7, £10.50, £14, £17.50." I am doing it in time with the 1950s American music they are playing. I take another sip. Yum.