There are lots of animations on the Web of planet collisions. In most, the planets maintain their (almost perfectly) spherical shape and their surface features right up to the point of impact. In some, the surface of one or both planets begins breaking up shortly before impact. This is due to tidal force, the differential between the near and far side of the object being acted upon by gravity. According to Wikipedia,
the Roche limit is the distance from a planet at which tidal effects would cause an object to disintegrate because the differential force of gravity from the planet overcomes the attraction of the parts of the object for one another.
In this National Geographic video, not only does the larger planet's crust begin to break apart seconds before impact, it even appears to bulge outward (taking on an ovoid shape?) to meet its collision partner a fraction of a second before impact. (Shouldn't it be the smaller object that breaks apart first, as in Wikipedia's visualization of a body crossing the Roche limit?)
The Roche limit applies only to
the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction
[s]ome real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength (that is, they are solid and not easily pulled apart). In extreme cases, objects resting on the surface of such a satellite could actually be lifted away by tidal forces. A weaker satellite, such as a comet, could be broken up when it passes within its Roche limit.
[i]t is also worth considering that the Roche limit is not the only factor that causes comets to break apart. Splitting by thermal stress, internal gas pressure and rotational splitting are a more likely way for a comet to split under stress.
We saw this happening in the spectacular pictures of comet Shoemaker-Levy disintegrating as it plunged into Jupiter.
And now for the really stupid part of my question. Why does the fracturing due to tidal effects apply only to celestial bodies and not to every macroscopic object?
What I'm asking is, if I hold a raw egg in each hand and move them towards each other, why does the mutual gravitational attraction and the front-back differential not make one or both eggshells fracture?
Doubtless the answer to my question is already contained in the Wikipedia articles if only I read them right. Does it have to do with the fact that surface area does not grow at the same rate as volume increases? Is the ratio of tensile force to gravitational self-attraction much greater for small objects than for planet-sized ones? Or is my error even more elementary?