to tap / to knock (against sth hard) / to knock (mud from boots, ashes from a pipe etc)
磕
=
盍
+
石
:
Karl Marx (k) knows he has to put out his cigarette before he can enter the elevator (e1) and his first instinct is to put his cigarette out by tapping (磕) it on the ancient stones (石) in front of the elevator (e1). He thinks "why not" (盍), but first makes a list with pros and cons (盍) to find the right solution to his problem.