Fill a clean, dry glass with lukewarm water. Lukewarm, so you don't get condensation of water from the air on the outside of the glass. Make sure your hand is completely dry, then pick up the glass and look down into the water, at the inside walls of the glass. You'll see a mirror-like reflection of the bottom. What you won't see is your hand holding the glass. You might see fingerprints here and there, but not much more. Wherever there's air between the glass and your skin, light is reflected back into the glass without ever reaching your skin.
Now dip your hand in water and pick the glass up with wet fingers. Look at the inside of the glass again and you'll see your fingers.
When your hand is wet, water fills the tiny gaps between skin and glass, so light from inside the glass can travel all the way out to your skin and back, allowing you to see your skin.
This not only demonstrates something about optics, but it shows why solid objects don't usually stick together: dry surfaces actually touch in only a few places.
Here's where glue comes in. It fills the gaps between objects and keeps them filled. Then the objects are held together by attraction between molecules, which is very strong if a lot of molecules are involved.
Water fills gaps in some materials, but it's not good glue because it runs out too easily. To glue your hand to the glass, you'd need either a stiff substance like the adhesive on duct tape, or something that hardens, like epoxy glue.
So, solid objects that seem to be in contact are actually separated by a lot of air. The job of glue is to fill in the gaps and keep them filled.
