Shells sound like the ocean because the curved shape of a seashell amplifies the ambient noise, or background noise, that normally is too quiet to be heard. This occurs because air bounces within the shell and makes sound, which varies in pitch depending on the size of the shell. A small seashell might produce a higher-pitched, more ocean-like sound than bigger shells because the air bounces around more quickly. Holding a glass or forming the mouth into an "O" shape and quickly tapping the cheek or throat will produce the same type of sound effect as the shell because it causes air to vibrate within a tight space.
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