A new plan for probabilistically requiring that people refill things—no more “but there’s one drop of milk left in the carton!”


It is generally understood that a person who finishes the last amount of something (e.g. milk, soy sauce, driving a shared car until the tank is empty) is also responsible for refilling the substance.

The issue:

This system is frequently gamed by the lazy, who will leave a tiny amount remaining so as to not have to refill the container (e.g. “But there’s still one more drop of milk in the carton!” or “There’s still some vapor in the car’s gas tank!”).

The underlying problem is that the expectation is that a person is free from obligation unless they consume the very last drop of something.


We can fix this by adding a sensor to each eventually-needs-refilling container.

Let’s use a refillable soy sauce bottle as a concrete example:

  • A soy-sauce-remaining detector (a floating ball) is added to the bottle (Figure 1).
  • Every time the soy sauce is poured, there is a chance that the bottle will light up and demand that the user refill it.
  • This chance isn’t uniform; when the bottle is 50% full or more, the chance is 0%. But as the bottle is emptied, the chance that a person will be called on to refill it increases.

Since it’s impossible to predict exactly when the bottle will need refilling, there’s no easy way to game the system.

Fig. 1: This “probabilistic refill” soy sauce bottle will unpredictably demand that the user refill it. The chance of being called on to refill the container increases as the container is depleted: at left, we see that a user would only have a 2% chance of being called on to refill the half-full bottle. The user is notified that a refill is needed by the flashing “RE-FILL” light on the lid (rightmost image).


Currently, this system just flashes a light on the item that needs refilling, but it could also snap a photograph of the offending user and—if the container is not refilled—upload it to a “you have violated the social contract” web site for public shaming.

PROS: Brings harmony to all shared-living situations.

CONS: Might be awkward if you use the last soy sauce during an earthquake and you can’t get any more for a while, so you’re stuck trying to survive while a beeping soy sauce bottle lid is threatening to publicly shame you. On the other hand, this is kind of the future we signed up for, right?