It was always things that seemed benign, but in hindsight were pretty discouraging. Mind you I'm in my 40's now but I remember in high school being scheduled for a class that taught basic logic and being unregistered and sent to the secretarial course - without my having been asked. Because girls weren't supposed to take that class anyway.The teacher "knew" it was a mistake because I was the only girl in there. This was mid to late 1980's.
What changed it all for me was actually having a computer at home and wanting to know more and do more. And having a husband who knew as little as I did - we tackled quite a bit together although I think learning C++ may have contributed to our divorce.
At any rate, I'm just saying, it's important to just let girls be and discover and grow. You'd be surprised how many girls are getting the same messages in 2015 that I got in 1980-something. If only everything "for us" wasn't pink or purple.