A classic American sentimental ballad often transcribed or adapted for piano. It is distinguished by its soaring, emotional melodic line and effective use of rich harmonic progressions.
Focus on the cantabile phrasing to ensure the melody projects over the harmonic accompaniment.
Comparable difficulty and character — good candidates for what to learn next.
'Who Knows?' is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'Who Knows?' lasts about 4 minutes.
'Who Knows?' is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Ballade by Emil Śmietański, Ballada by Jonatán Zámbó, Prelude in E minor, No. 4 by Frédéric Chopin — see the full similar-pieces list on this page.