“One of the most important tasks I have for you” the innkeeper told his new the seller boy “is keeping track of how much ale and wine we have.” He waved at the row of tall oak barrels along the cellars wall. “You need to tell me when we get halfway down a barrel so I can start prepping its replacement. I don’t want you using any filthy sticks in my beer, mind. You can take the lit off and look, nothing more. Ah, don’t look like that, lad! Telling halfway is easy!”

So, with no measuring tool available (other than your eyes) and no indicators inside the barrel, how would you work out when the barrel is half empty?