Left: foot 足 (knee cap 口 and footprint 止), right: Variant of: collapse 塌 (You will come back to earth 土, when you want (to reach) the sun 日 with your flight 羽, whereat you collapse. [HERE: with foot 足 instead of earth 土]
This radical was developed from a circle with a central dot as the image of the sun. As you can hardly draw circles or dots with a brush, it became the current rectangular shape.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!
