why do sz, ś, and si all sound almost exactly the same??

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why do sz, ś, and si all sound almost exactly the same??
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Ohohoo my day was even lazier than yours,woke up on afternoon ,helped mom making lunch,didnt eat lunch& sat in the living room,then i got tired probably because i didnt eat anything since last night so i made some cereal then i felt even sleepier so i lied down on my bed,took a 10 minutes actual nap,spent 2 hours doing nothing but pretending to be asleep , took out the book im supposed to study for tomorrow's test,read a few pages,ate dinner then here i am lying on my bed doing nothing again /:
yup you win. lazy days are so good but so bad at the same time >.<
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