What’s the most complicated system you’ve used without ever knowing how it actually works?

Was thinking about this the other day. Things so many people encounter on an everyday basis, things we interact with where we have simply memorized 'if I do x, that's what y is going to do', without an understanding of what is happening under the hood.

This idea is analogous to DNS (a bit like your phone knowing where to look for a tower), or how your phone seems to predict what word you're using next. Most people use them frequently and are completely ok with them, but if you asked about them they would just shrug their shoulders when you asked about how they work.