A pleasing little philosophy puzzle:

If there’s a sentence that’s guaranteed to be false in any context, surely it’s this:

“I am not here now.”

But this very phrase is played on millions of answering machines and voicemail systems every day, and we all understand it to be true. I, here, and now are indexicals, words whose meanings change with the circumstances of their utterance. Here each seems to make a rather uncertain reference, and the resulting sentence on its face cannot be true, yet we all understand it readily. How?

(Jonathan Cohen, “Indexicality and the Puzzle of the Answering Machine,” Journal of Philosophy 110:1 [2013], 5-32.)