The radical tsuki stands for moon or month when used at the right side of a character. However the radical niku 肉 that stands for body or body-part is in many cases simplified to 月 and thus looks equal as "moon".
The radical niku is typically at the left side of a composition and drawn as: 月, which looks equal to: "moon, month 月" but still has the meaning 'body part'.
Tip: Go to the radicals that are contained in this character and learn all the characters with this radical systematically!
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!
