I KISS it (Keep It Stupid Simple) by saying something like this:
You ever taken a standardized test before like an SAT? You know how your brain feels after? This is how my brain feels after a day of coding where I am intensively solving problems.
The audience here makes a difference. Are we talking about explaining it to some random person I just met who has never done anything technical at all? Or explaining it to a physicist/biologist/etc...? I would take two entirely different approaches. For a non-science person who I can tell isn't as strong on the "intellectual" side, or maybe who is but whom is just very uneducated, I will KISS it.
For a scientist or engineer, I will go into much more detail but I have a feeling that a scientist or engineer will probably mostly know how programming is exhausting and thus not have to ask the question.