The Riddle of the Sphinx

I read some sad news the other day about Terry Pratchett, who has been diagnosed with Alzheimer's disease. Ever since I first started reading them in high school, Pratchett's discworld novels have been my favourite comfort books- they're funny, clever, often thought-provoking, and are enjoyable to read time after time.

Here Pratchett's version of the riddle of the Sphinx, from one of my favourites, Pyramids:

'The answer is: "A Man",' said the Sphinx. 'Now, don't put up a fight, please, it releases unpleasant chemicals into the bloodstream.'
Teppic backed away from a slashing paw. 'Hold on, hold on,' he said. 'What do you mean, a man?'
'It's easy,' said the Sphinx. A baby crawls in the morning, stands on both legs at noon, and in the evening an old man walks with a stick. Good, isn't it?'
Teppic bit his lip. 'We're talking about one day here?' he said doubtfully.
There was a long embarassed silence.
'It's a wossname, a figure of speech,' said the Sphinx irritably, making another lunge.
'No, no, look, wait a minute,' said Teppic. 'I'd like us to be very clear about this, right? I mean, it's only fair, right?'
'Nothing wrong with the riddle,' said the Sphinx. 'Damn good riddle. Had that riddle for fifty years, sphinx and cub.' It thought about this. 'Chick,' it corrected.
'It's a good riddle,' Teppic said soothingly. 'Very deep. Very moving. The whole human condition in a nutshell. But you've got to admit, this doesn't all happen to one individual in one day, does it?'
'Well. No,' the Sphinx admitted. 'But that is self evident from the context. An element of dramatic analogy is present in all riddles,' it added, with the air of one who had heard the phrase a long time ago and rather liked it, although not to the extent of failing to eat the originator.
'Yes, but,' said Teppic crouching down and brushing a clear space on the damp sand, 'is there internal consistency within the metaphor? Let's say for example that the average life expectancy is seventy years OK?'
'OK,' said the Sphinx, in the uncertain tones of someone who has let the salesman in and is now regretfully contemplating a future in which they are undoubtedly going to buy life insurance.
'Right. Good. So noon would be age 35, am I right? Now considering that most children can toddle at a year or so, the four legs reference is really unsuitable, wouldn't you agree? I mean, most of the morning is spent on two legs. According to your analogy-' he paused and did a few calculations with a convenient thighbone- 'only about twenty minutes immediately after 00.00 hours, half an hour tops, is spent on four legs. Am I right? Be fair.'
'Well-' said the Sphinx.
'By the same token you wouldn't be using a stick by six p.m. because you'd be only, er, 52,' said Teppic, scribbling furiously. 'In fact you wouldn't really be looking at any kind of walking aid until at least half past nine, I think. That's on the assumption that the entire lifespan takes place over one day which is, I believe I have already pointed out, ridiculous. I'm sorry, it's basically OK, but it doesn't work.'
'Well,' said the Sphinx, but irritably this time, 'I don't see what I can do about it. I haven't got any more. It's the only one I've ever needed.'
'You just need to alter it a bit, that's all.'
'How do you mean?'
'Just make it a bit more realistic.'
'Hmm.' The Sphinx scratched it's mane with a claw.
'OK,' it said doubtfully. "I suppose I could ask: What is it that walks on four legs-'
'Metaphorically speaking,' said Teppic.
'Four legs, metaphorically speaking,' the Sphinx agreed, 'for about-'
'Twenty minutes, I think we agreed.'
'-OK, fine, twenty minutes in the morning, on two legs-'
'But I think calling it "in the morning" is stretching it a bit,' said Teppic. It's just after midnight. I mean, technically it's the morning, but in a very real sense it's still last night, what do you think?'
A look of glazed panic crossed the Sphinx's face.
'What do you think?' it managed.
'Let's just see where we've got to, shall we? What, metaphorically spekaing, walks on four legs just after midnight, on two legs for most of the day-'
'-barring accidents,' said the Sphinx , pathetically eager to show that it was making a contribution.
'Fine, on two legs barring accidents, until at least supper-time, when it walks with three legs-'
'I've known people use two walking sticks,' said the Sphinx helpfully.
'OK. How about: when it continues to walk on two legs or with any prosthetic aids of its choice?'
The Sphinx gave this some consideration.
'Ye-ess,' it said gravely. 'That seems to fit all eventualities.'
'Well?' said Teppic.
'Well what?' said the Sphinx.
'Well, what's the answer?'
The Sphinx gave him a stony look, and then showed its fangs.
'Oh no,' it said. 'You don't catch me out like that. You think I'm stupid? You've got to tell me the answer.'
'Oh, blow,' said Teppic.
'Thought you had me there, didn't you?' said the Sphinx.
'Sorry.'
'You thought you could get me all confused, did you?' The Sphinx grinned.
'It was worth a try,' said Teppic.
'Can't blame you. So what's the answer, then?'
Teppic scratched his nose.
'Haven't a clue,' he said. 'Unless, and this is a shot in the dark, you understand, it's: A Man.'
The Sphinx glared at him.
'You've been here before, haven't you?' it said accusingly.
'No.'
'Then someone's been talking, right?'
'Who could have talked? Has anyone ever guessed the riddle?' said Teppic.
'No!'
'Well, then. They couldn't have talked, could they?'
The Sphinx's claws scrabbled irritably on its rock.
'I suppose you'd better move along then,' it grumbled.
'Thank you,' said Teppic.
'I'd be grateful if you didn't tell anyone, please,' added the Sphinx, coldly. 'I wouldn't like to spoil it for other people.'