Phrase: He mouths all of the words to me. He's so cute!

[Mickey]

English Language Tests, Intermediate level

ESL/EFL Test #721 "Real Life: Using Body Parts as Verbs (1)", question 5

When my three-year-old son is trying to whisper, he doesn't make a sound. Instead, he ......... all of the words to me. He's so cute!

(a) mouths
(b) faces
(c) heads
(d) arms

English Language Tests, Intermediate level

ESL/EFL Test #721 "Real Life: Using Body Parts as Verbs (1)", answer 5

When my three-year-old son is trying to whisper, he doesn't make a sound. Instead, he mouths all of the words to me. He's so cute!

Correct answer: (a) mouths

Your answer was: correct
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I don't understand this sentence. Can someone explain it to me, please?

[Mister Micawber]

Which words confuse you?
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Native English teacher at Mister Micawber's

[Mickey]

Would you explain "he mouths all of the words to me", please?