In life I've met with many things everyone else knew but me: how to pronounce "derriere" and "decollete"; that people are born naked (the shock!); that bottled mayonnaise and salad dressing differ while looking the same; how to light matches; that not all plants are perennial; that screws tighten when turned rightward; that salaries are negotiable and the first offer is a lowball; that canned tuna must be drained (we never ate canned tuna; the kids I babysat showed me how); how to tie shoelaces without making two loops and crossing them. From age 6 to 55 I walked around with shoelaces tied "backwards" and ignorance was bliss.
That applies to my first telescope lesson. The eyepieces ordered, of good quality and standard size, were a bit too big to fit in the tube you put your eye to. I tried everything. I even unscrewed one lens from its base and dropped it into the barrel, and oops, down the tube it went, requiring me to strong-arm the 22-pound telescope upside down to get it out.
I would not give up on my free telescope with useless new $20 eyepieces; I just wouldn't! After a long search I learned a fact apparently everyone knows, so basic it's not even listed in specifications: older cheaper telescopes use eyepieces .965 [inches] across, and today's standard measures 1.25."
.965s are so obsolete that adapters are scarce, but I found and ordered one. From now on, though, I know what to tell anyone who fosters or adopts a homeless telescope.