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When you say the "ee" sound, your mouth naturally forms a smile: the muscles get tight and the mouth gets wider. When you say the short "i" sound your mouth should be completely relaxed, even hanging open a bit. Try to focus on the position of your mouth and the tightness of your muscles when you say these pairs of words:
sheep ship
feet fit
heat hit
meet mit
peach pit
speech spit
Your mouth should change a great deal when you switch to the next word.
