Shall I tell you now? :)
Alright, I guess I will.
Butter--melting on our electric griddle! It was all melting in one spot, so I took a knife and swirled it around a little--thus the design. Once I'd done that, I looked at the griddle and thought about what a great mystery picture this would be. :) It worked so nicely that even my parents didn't know what it was! (My brothers might not have known either, except that they saw me taking the picture.)
It was so fun to see all of your guesses and then to look back at the picture and actually see what you saw there!!! Thank you so much for your participation. I hope you enjoyed this as much as I did!