Sometimes it's not knowing the difference, but often it's a typo. I know that from my experience--I'm always typing the wrong one though I've known better since early elementary school. There/their/they're, on the other hand, I type wrong less often, but not never. The way the human mind is wired, it is easy to make homophonic typos, not just transposed pair deleted/inserted letter, or changed letter typos. In my case, I'm sure I'm playing the sound of the sentence in my head as I compose it and type it, and if something interrupts the chain of thought, say, changing my mind about a word I'm going to type, I may type a word based on its sound instead of its meaning. (The "sound the sentence in my head" also means I often place commas wherever there are pauses, even if some disagree that a comma "belongs" there or not). Incidentally, at the most basic level, I don't think in words--words are a level of abstraction above thoughts for me. I suspect I'd not be much of a mathematician if I were required to think of mathematical concepts in words--or musician either for that matter.

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Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

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