A man without a seat walks around the room looking for the next available stool by checking at the amount of sand in the hourglass encased in each stool.
Current occupant of the stool checks the remaining sand on the top bulb of the hourglass knowing that he can still use the stool for a little while, so he sits back.
Occupant of the stool realizes the top bulb is emptied. He worries about how others around the stool might have imposed negative impression upon him, so he stands up and prepares to leave.
Knowing the availability of the stool in advance, the seatless man quickly walks up to the vacant stool and become the next occupant.