How it works:

The SmartShelf is one that can measure the collective weight of the items on the shelf. It tracks the weight of the shelf over time. As an item is removed and replaced, it checks the difference in weight between when the item was picked up and when it was replaced. If that weight is less than or equal to 5% of the item’s weight before it was picked up, it will yell at you through embedded speakers inside the fridge. The percentage difference can be calibrated through a knob under the speaker (not shown in illustration).

In peudo-code it might look like:

currentWeight = measure(currentWeight);
newWeight = measure(currentWeight);

if (newWeight < currentWeight ){  //if true, something's been removed
weightWithItemMissing = newWeight;
itemWeight = currentWeight - weightWithItemMissing;
//take new measurement
newWeight = measure (currentWeight);

if (newWeight > weightWithItemMissing){ //something was put back
if (newWeight - weightWithItemMissing <= 5% itemWeight) {
yellThroughSpeakers("Why Don't You Finish It?!");
}
}
}

(This was created for an assignment in which we had to imagine and describe a fantasy device.)