I'm probably opening myself up to ridicule here but I've been meaning to do this for a while.
I thought it might be fun (and we might learn some stuff too) to post things that you've only learned recently that were in hindsight a bit obvious. I'll start off with two:
I only discovered recently that when you pull up a shutter blind you have to pull the cord in a specific direction. Pull to the right and down to keep them up and pull to the left and down to release them. I had to just go and check that though on my own blinds
I also only noticed recently that you can sometimes see the moon during the day and sometimes not at all at night (because it's on the other side of the world in their day). In hindsight very obvious because of it's orbit around the earth but I'd never bothered to notice!