Like the title says, please explain surface tension to me. I poured a drink tonight and was curious about how it works.
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Molecules in a liquid have weak bonds to the molecules around them. These bonds aren’t as energetic as full chemical bonds, but they do still hold things together. Within the liquid, the bonds all point in random-ish directions, so the individual molecules of liquid just sorta hang out.
But at the surface of the liquid, the molecules are only bonded to molecules inside the liquid, and not to the molecules outside. This is a higher-energy state for the liquid molecules, so they don’t want to be in that state. In other words, the liquid as a whole wants to minimize the number of molecules exposed to the outside world or, in simpler terms, to minimize its surface area.
For a liquid in a container, this results in a flat surface (except right around the edges of the container, where it can bend slightly). For a liquid bulging out of a container (as with your beer) or e.g. floating as a droplet in midair, it results in a curved shape (in the case of a droplet, a sphere).
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