Outside frame 囗, but here: mouth 口, inside: old 古 (= tomb stone). [Some things like the meat of old animals or full-grown vegetable become hard with age.]
Mnemonic
In the mouth old things taste: hard. (In contrast: The young/immature 未 is in the mouth 口 with a taste 味)
Outside a frame 囗, which stands as a placeholder for things that become hard with age 古, such as the meat of old animals or old vegetables/roots. In other words: "things" that are old are also hard.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!
