When the rice is inside the bottle, there are grains next to one another, but there is a little bit of space, an air pocket, in between each grain and its neighbor Which allows the chop stick to slide out of the bottle. When the rice is packedmore tightly, the friction between each piece of rice and its nearest neighbors increased. At some point, the friction between the chop stick, rice and bottle was so great that it is strong enough to resist slipping and pick up the bottle.