This tool is more effective for dogs who travel farther and are more visible, although it can also be used for cats. Keep it simple with large lettering and few descriptive words. A photo taped inside the window is optional.
These markers are sold in some stores and online and are typically used to write "Graduated from High School" or "Just Married" on car windows. Be sure to use markers designed for this purpose so they will wash off easily and not damage the vehicle. The windows must be clean and dry for the markers to work properly.
In most areas it is legal to decorate the rear window and the rear side windows, but not the windshield or front passenger windows. Observe local laws and be safe.