“You know, building muscle takes time.”
You are a really physically fit person who works out at a gym several times a week. Your friend is complaining that she's been lifting weights but her muscles haven't gotten any bigger. You say this because you think that she just needs to continue doing it.
You know, building muscle takes time.
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You know, (sentence)
You actually start a sentence with "You know," when you want to give someone information that you think they don't know.
Adding "You know," makes you sound intelligent and informed. But people can get annoyed if you use it too much or use it to talk about a subject that you don't have special knowledge of.
You wouldn't use "You know," this way in writing. When you pronounce this, you should stress the word "know":
You know, building muscle takes time.
build muscle
To "build muscle" means to get bigger muscles. This is the way that people who are interested in fitness and weightlifting talk about getting more muscle.
(doing something) takes time
When you say that "___ing takes time", it means that someone needs to spend a long time doing it to get results. For example:
Figuring out what you want to do in life takes time.