??
Here’s a SD column on How does glue work? More or less what you are asking.
“We’re not entirely sure.”
The simple answer is adhesion…which just answers the question by giving it a label. The more informed answer is a topic one could read a series of technical handbooks about and still not thorough knowledge of the subject. Like tribology (the study of lubrication and friction between solid surfaces in contact) there are a large array of disparate phenomena that contribute to the “stickiness” between surfaces. These can be essentially broken down into four different categories:
[ul]
[li]Mechanical adhesion: the surfaces mate together with some mechanical locking feature (Velcro and Metalklett), or one substance flows plastically into another (wax or peanut butter in fabric) but maintains its macrostructural distinctiveness (i.e. remains as a continuum of a single substance)[/li][li]Interface diffusion: the materials at the interface flow into one another at a molecular or atomic level; some types of polymer binding, intergrain interfaces in polyphase metallic substances[/li][li]Chemical adhesion: ionic, covalent, or hydrogen electrochemical bonds form between the atoms or molecules on the interface resulting in bond forces; this is the primary (or at least, preferred) bonding method for most common adhesives and epoxies as well as paint and other protective surface coatings[/li][li]Electrostatic attraction: opposing charges or van der Waals attraction create attractive electromagnetic forces, but the bond is not a “permanent” set like a chemical bond; electromagnets, obviously, or the way dust collects on the body of a car (or rather, the ionized substances deposited on the body by rain or ionized water)[/li][/ul]
Mind you, this is just a very general categorization, and most real world bonds will be a combination of the above; for instance with paint, the surface is typically opened or roughened by sanding or sand-blasting to create more surface area and open up the pours (especially with wood or porous polymers), which both gives more surface area for the chemical bond between the primer and substrate, and gives some mechanical resistance to sliding decohesion (rubbing and abrasion). The paint that goes on top of the primer is a combination of chemical bonding and interface diffusion, and the lacquer or varnish that goes on top of the paint to form a final protective layer is the same.
On the other hand, when your toast falls butter-side down and picks up the dust and cockroach eggs that are on your unmopped kitchen floor, that brunette is a combination of mechanical locking and a bit of chemical attraction. The sticker that you can never completely pull off of the DVD case? That’s almost purely chemical, although I’ll swear they somehow weave it into the case. And the dollop of mayonnaise that fell on your pant leg right before your first date with that gorgeous six foot Swedish? Almost all mechanical unless you are wearing some kind of really cheap synthetic or unsealed cotton, and even then it usually takes a good solvent to really bond in the stain securely. (Several substances in food, however, are excellent solvents, so make certain to go with something non-messy for the date.)
I doubt that this really answered your question in adequate depth, but to do so would take at least a several page column and would still only touch superficially on what is one of the most complicated and nuanced topics in mechanics. Compared to this, gyroscopic motion and celestial mechanics are child’s doddle.
Stranger
Don’t forget the van der Waals force - example
I love stickum.
Er, that needed the proper, Bud Light endorsement:
I love stickum! Hic!
What makes sticky things sticky?
Children in my experience.
I have a pretty good idea of what makes my fingers sticky. Need to experiment a little more though. 
[quote=“Stranger_On_A_Train, post:3, topic:516004”]
[li]Chemical adhesion: ionic, covalent, or hydrogen electrochemical bonds[/li][/QUOTE]
That’s a very broad category that covers the fusing of molecules as well as various forms of electrical attraction. The electrical attractions themselves deserve a whole chapter.
I’m not certain what you mean by “fusing” of molecules; I assume you are referring to cross-linking of polymers, but this doesn’t directly result in gross adhesion, but rather causes alteration of conformal configuration of molecules that makes them more amenable to the previous stated forms of adhesion, or otherwise stabilizes a copolymer matrix (for instance, binders within a propellant grain or stabilizing organic molecules by a preservative). All of the stated categories are, as previously indicated, merely broad representations of the myriad of detailed phenomena that result in what we see as “stickiness”.
Stranger