van der Waals stacking
Set two flat magnets face to face and they cling; slide a book's pages and they part with a whisper. Layered crystals are held together in just this gentle way. Within each atomic sheet the atoms are locked by strong chemical bonds, but neighbouring sheets are attracted to one another only by the faint, universal van der Waals force — the same weak stickiness that lets a gecko walk a ceiling. Van der Waals stacking is this loose piling-up of strongly-bonded sheets, one on the next, held only by that weak interlayer glue.
Graphite is the textbook case. Each carbon sheet is a honeycomb with bonds about 0.142 nm long — extremely strong. But the sheets sit a full 0.335 nm apart and only van der Waals forces hold them, in an ABAB sequence where every other sheet is shifted. That is why a pencil writes: the lead's sheets shear off in slippery flakes across the weak gaps, while each flake itself is a tough, intact crystal. The same two-tier structure — strong in-plane, weak out-of-plane — appears in hexagonal boron nitride, the dichalcogenides, and every 2D material.
The most exciting consequence is freedom. Because the interlayer bond is weak and non-specific, the atoms in one sheet do not have to line up with the atoms in the next, so you can peel single sheets off and re-stack completely different materials in any order — graphene, then boron nitride, then a dichalcogenide — into designer van der Waals heterostructures. You can even rotate one sheet by a small twist angle, which produces a moire superstructure. This is a wholly different way to build a crystal: not by growing one lattice, but by assembling sheets like a deck of cards.
Drag a pencil across paper and you deposit graphite: the strong carbon sheets stay intact, but the weak van der Waals bonds between them give way, so whole sheets slide off and smear onto the page. That everyday act — writing — is a demonstration of van der Waals stacking failing between layers while the layers themselves survive untouched.
Strong bonds within sheets, weak van der Waals bonds between them: sheets cleave and re-stack freely.
Van der Waals forces are genuinely weak, but not zero — they still bind the crystal firmly enough that bulk graphite is a solid, and the stacking order (ABAB in graphite) does set real properties. Because the layers need not register with one another, van der Waals epitaxy sidesteps the strict lattice-matching that ordinary epitaxy demands.