Hey, I came across many mind-blown kind of facts when studying theoretical CS, such as this one:
A pushdown automaton cannot tell which strings are of form "a^n b^n c^n", but it can tell exactly which ones are not.
That's like if someone wasn't able to tell which sentences are in English, but could tell you exactly which ones are not.
What are some facts like this, that you only discover in higher-level math, and people outside the field don't usually know? Try to also provide ELI5 simplified explanations :)