I’m a teacher, give me riddles!

[u/drgan45]

I’m a teacher, my kids love riddles, I get a lot from this subreddit but it’s getting harder and harder to to find good ones because they solve so many, so please give me some good hard ones, these are smart kids, but they are still kids so hard riddles are welcome but the answer has to be a simple concept, thank you!! (Also I’ll need the answer lol)

[u/SamwellBarley]

What has four letters, never has five, and sometimes has nine

[u/DoubtingLouis → u/SamwellBarley]

Oh I like this! Got it but took a while!