A Linux SysAdmin walks into a Chemist shop.

“Uhmm, Ephedrine please”

“We can’t sell you that, it’s a controlled substance”

“Sudoephedrine”

“Certainly Sir, here you go”

Look, most people won’t get this joke. That’s ok, it’s targeted at a very narrow audience.
Don’t worry about it, there’s plenty more to read on this site.

The grandaddy of all Linux jokes, due to its worldwide fame amongst the geek community is this one from xkcd: