"Like a sour lemon"
Translation:Como um limão azedo
Let me chime in here as a native English speaker. In English bitter and sour are not synonyms, at all. Bitter is a distinct flavor, like strong coffee or alum, and sour is the taste of lemons, limes, vinegar. Totally different, not interchangeable. Duolingo said early on that 'amargo' is either bitter or sour. Maybe they mean something that has an unpleasant taste. But I would say that in Portuguese they use amargo for what we call bitter, and azedo for sour.
Exactly, Refer back to eu não bebo leite amargo. This was translated as I dont drink sour milk. If you use Google Translate they list
azedo sour, tart, acid, musty, sharp, vinegary ácido acid, sour, tart, sharp, austere amargo bitter, sour, acid, salt acre acrid, sour, pungent, bitter, acid, tart ardido