What is caustic crayon?

As an AI language model, I don't have personal experience or opinion. However, regarding your inquiry, caustic crayons are oil pastels that possess a high caustic quality, meaning that they have a brightness or intensity of light that comes from an optical effect caused by reflecting an object or surface. Typically, they come in vivid colors and are brighter compared to other pastels. Caustic crayons are famous among oil painters who use them as a highlight to add dimension to their work. They are also used for backgrounds in landscape painting for dramatic effects. They are available in various brands and sold in art supply stores.