How is man like Hevel?
Radak: Hevel is something that does not last, like breath.
Why does it say "Yamav k'Tzel Over"?
Rashi (from Koheles Rabah 1:3): Shlomo said "v'Ya'asem ka'Tzel" (Koheles 6:12); he did not specify whether this is like the shade of a date tree or wall, which are fixed. His father already explained "k'Tzel Over" - like the shade of a flying bird (it is only for a moment) 1 .
Radak: They are like shade that passes quickly when the sun moves.
Sha'arei Teshuvah (2:19): Since a person does not when he will die, efforts for [physical matters for] the future [could be] paining himself over a world that is not his. Do not be pained over future afflictions, for (Mishlei 27:1) "Lo Seda Mah Yeled Yom" (Sanhedrin 100b).
