Most of the human brain's energy consumption is not dependent on how hard you are thinking

https://cordis.europa.eu/article/id/442230-can-you-burn-calo...

I think differential equations are a funny example because they range from "turning a crank" to solve some simple ones to a very difficult search for a combination of clever hacks and techniques from a large and poorly organized toolbox... e.g., there's a lot of questions over whether the typical undergrad course in differential equations is well organized.