Moving to a position on the surface of something.
Formed in Middle English from "on" + "to" to make a clearer distinction between static position ("on") and movement ("onto"). It became standard to show motion toward a surface.
English often builds new prepositions by stacking old ones, and "onto" is literally "on" plus "to." The language is quietly solving a physics problem: are you already on the surface, or are you moving there?
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