Synonyms of 'sit in on something'
▷ See sit
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If you observe a person or thing, you watch them carefully, especially in order to learn something about them.
He studies and observes the behaviour of babies.
If you watch someone or something, you look at them, usually for a period of time, and pay attention to what is happening.
The man was standing in the doorway watching him.
Thesaurus for sit in on something from the Collins English Thesaurus