Why can't I hear clearly when I speak with a mask on?

In the oral exam during the epidemic,

You need to wear a mask throughout the process.

Therefore, the question "can't explain clearly" often appears.

A funny scene of "not understanding".

However, when you take off the mask, even if you plug your ears,

You can also guess roughly by the shape of the mouth.

Why is this?

This issue's copywriter: Super love Belle

Design: Nell Mommy Mommy

Editor: Zhongguancun Qiong Yao

Review: Spicy Shredded Chicken

References:

Maxwell puzzle solving project, what did you say? You are wearing a mask and I can't hear you clearly

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Source: China Science Expo