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Why does the share price tend to fall if a company's profits decrease, yet remain positive?
Let's say you see a café. You're looking to buy a café so you walk into one and ask the manager how much profit he makes in a year. He says $N and you walk out and think to yourself, "I'd be willing to pay $500,000 for this café." You arrange to meet again to discuss purchasing the business (and he's looking for someone to purchase it).  You go into the store again the following day and the manager says, "Sorry, I told you we make $N. I've checked the numbers and it's actually only $0.8N (20% lower than what you thought)." Are you still willing to buy the café for $500,000 as well? No, of course you're not.  I think that this is a sufficient analogy to public companies.