OK, this one is almost too simple to mention, but it was fun so here you are. Get some watercolor paper and a white crayon. Draw a spider web with the white crayon (in secret!) and then tell the children to just paint a midnight sky. Tada! My children were so surprised that a spider web appeared (because you know, the watercolor paint won't stick to the crayon lines and I suppose you could go into the chemistry of all that if you were sufficiently on top of things)
My children then decided to create their own designs and I ended up with this from my daughter.
Happy Monday friends. 'Tis a busy one!