but does anyone know where "teaching your grandmother to suck eggs" comes from?

This is what I found, from the following site: http://makeashorterlink.com/?Z17F52EB which is a Welsh / English dictionary (of sorts).

Welsh: yr oen yn dysgu'r ddafad i bori ("the lamb teaching the sheep how to graze") said of the inexperienced presuming to know better than experienced people, children who think they know better than the parents; 'teaching one's grandmother to suck eggs'

I'm not sure that this is really explaining much, merely giving another phrase which means a similar thing. However, here's the link for what Quinion has to say: http://www.worldwidewords.org/qa/qa-tea1.htm which gives a bit more background.

See the shame about having this be a 'new posters' thread is that tsuwm is probably the person who knows the other places to look for answers to these kinda questions, and he probably ain't here looking at this thread. Sigh... tsuwm (or any other 'oldies') if you're reading, willing and able, wanna PM me or dxb with any insights you might have? Ta!