acido can mean 'sour' or 'acid'

Depends on context, I would think. May be one of the following usages: adverb:

as----como, mientras, ya que, cuando, lo que, donde ;
like----como ;

preposition:

as----------como, por, para, en calidad de, a nivel;
like---------como, propio;
by way of-----a modo de, como

so, could possibly be like a tart/sour/acid lemon or as a tart/sour/acid lemon

I deed, without more information both are correct: "I eat a sour lemon" and "Like a sour lemon", since "(yo) como" means "I eat".

My question wasn't the meaning of acido, but the meaning of como. Why isn't this I eat a tart/sour/acid lemon.

I still don't get it. Is it because I eat lemons??? Is there something in that sentence that makes the context NOT I eat?

Jeanine, what is missing is where you came across the sentence " Como un limon acido,' Were you translating a document or was it in a lesson? It's necessary to know how the sentence (or phrase possibly) was being used. 'Como' has different meanings depending on how it is used. Was there a sentence preceding the one in question that would help explain the context? If so, what was it?

This was a question in duolingo. So there was no context.

I'm confused. You asked the question':"Why isn't this I eat a tart lemon?' regarding "Como un limon acido." It could be 'I eat a tart lemon.' Something or someone apparently led you to question that possibility. But, it could also be the response to a question 'what does it taste like?' The answer could be 'Como un limon acido.' (like a tart lemon), where 'como' would mean 'like.' Context matters. I hope this helps.