You pour coffee into your mug each morning. The liquid level rises smoothly inside the ceramic walls, staying neatly contained as you fill it.
The surface climbs toward the rim. You tilt the pot more carefully, keeping the brew from touching the edge.
The exact moment arrives when the liquid height exceeds the mug's rim.
Before that point, all the coffee remains held within the mug's interior. Immediately after, droplets break free and flow over the side, landing on the saucer below.
The mug shifts from a contained vessel to one actively spilling its contents. This marks the crossed boundary, visible in the sudden appearance of liquid outside.