Graphene’s electrons can slosh back and forth. One moment they accumulate in center (leaving the corners slightly positive). Another moment they accumulate in the corners (leaving the center slightly positive). To view this sloshing, repeatedly click this button:
Now consider the effect on an adjacent graphene layer: Minus charge accumulations on the top layer will draw plus accumulations below on the bottom layer (and vice versa). To show / hide charge sloshing on the bottom layer, click this button (and then cycle the Step Forward button):
These fleeting complementary charge distributions will draw the two layers together in a process called "van der Waals" bonding.