Here's a mystery from my daily life. Every night at 9 and 11 pm, the people who live on the floor under my studio always open the window to the veranda, and bang on something 'wooden' exactly twice (Never three times). "Knock Knock!". Then they slam the window shut again.
This "Knock Knock" sound reverberates through the night sky. What could they possibly be doing? And this happens every night! (To Be Continued in Volume 38)