When is a riddle not a riddle?

When it's a joke...

The conversation went something like this:

HUMAN: Alexa, tell me a riddle please.
ALEXA:  What has four legs but only one foot? (Pause) A bed.
HUMAN: Alexa, please tell me another riddle.
ALEXA:  How many surrealists does it take to change a lightbulb?

Now this is a joke, isn't it? And when I was a kid, the answer to this, the punchline, was Fish. But times have moved on, clearly, because...

ALEXA:  Two. One to hold the giraffe and the other to fill a bath-tub with brightly coloured power tools.

Which is pretty funny, actually. But it's not a riddle, Alexa. Keep working on those algorithms, Jeff et al.