Cracks, as noted by Tom Nook, are an area of the ground that "appears unusual". Each day, a few cracks, which are sometimes called "starfish" due to their shape, will appear in the town. These cracks may hold fossils or Gyroids if dug up with a shovel. These fossils can then be sold to Tom Nook in Wild World and City Folk, or to Reese at Retail. Gyroids can also be sold or displayed in a house.
By using the shovel, any non-living item can be buried. This items include furniture and shells. For fruit and saplings (and on some occassions, Bells, depending on the game) these will result in a sapling.
Pitfalls randomly appear in cracks, caused by pitfall seeds. When the player walks too close, the crack will disappear and they will fall into the hole. If a tricky pitfall seed is buried, the crack will appear smaller than usual.
In Animal Crossing: New Leaf, 5 cracks appear on each day; 4 of them reveal fossils while the last one reveals a pitfall seed. If it rained/snowed the day before, there will be 8 cracks; the other 3 which reveal gyroids. A crack can also be seen once a time capsule (which a villager asks the player to bury some time ago) is ready to be dug out.