In the early 1970’s Kelly Hart devoted much time refining a technique for making animated movies that he eventually patented in the United States. He called this technique Fluid Art and he created a number of short animated films using it. Basically it employs a light box with a glass surface that is warmed by the lights. When colored grease pencils are used on the glass they melt and become fluid enough to manipulate with a paint brush or other implements. Animation is created by having a camera record small changes in the image as it is so manipulated.
Kelly has a YouTube channel devoted to this work where you can see it in action: www.youtube.com
Here are a few examples of this animation: