I know someone I really like, but sadly, that person has absolutely no experience in math or mathematical thinking above third grade mathematics (+, - are fine, but division already makes problems). This person is an adult and not stupid, but he never had any opportunity to get more educated and it worked for all his life like this.
Now, he asked me why you'd need proofs for anything ever, since it's enough to see that an equation works after testing it for a few values.
What very practical example could I use where an equation works for like the first 100 numbers in $\mathbb{N}$ or something like that, but fails afterwards? Something that doesn't need a long introduction into equations (since this is above his head), except for things like "? + 5 = 10, what is ?" ($x$'s needlessly complicate this since it "looks too mathy").