Bought some of theses http://www.crayola.com/browse-products/crystal-effects-window-markers.aspx
via ebay. I have only just had a play with them. My bedroom window looks like Jack frost has been to visit.
Nice video of them in action Crayola crystal effects window markers - YouTube
Nifty looking. I would have loved them as a kid.
I just can’t get enough of the crystals forming.
I MUST HAVE THESE! We will have so much fun. 
Oh squee! My nieces need these! My sister will shoot me but isn’t that what being an aunt is for?
Our house has two small walls that used to be exterior walls but became interior walls when we added another room to the house. But we left the two windows in place, so we have these two double-sash windows in our house. Anyway, our kids got these crystal window markers several months ago and they love them. They play a game where one kids stands on each side of the window and they take turns adding to a picture together from opposite sides of the glass.
Very cool skammer
adds to her wishlist
Ooooh, I can’t wait to play with some of those! I just renewed my stash of sidewalk chalk, too!
Must have gotten humid overnight because my lovely crystals have turned liquid. Oh well must force myself to play with them again 