Usually, the double verb 看看, when it follows another verb (i.e., V + 看看）means "try to V". Example: 这水饺很好吃，你吃看看。 These dumplings are delicious, try (eating) some. On the other hand, 看 usually means "to look at, to see." Example: 你看那幅画，很漂亮吧。 Look at that painting. It's beautiful, isn't it? Frank makes an excellent point, but I think 看 can also be used as an intransitive verb: The following sentences do not have a direct object following 看。 你看吧！ You see?! 你看着办吧！Literally, "Observe and do accordingly!" but actually means "it's up to you!" 你让我看嘛！Let me see, please! When 看看 is used alone (not following another verb), it also means "to look" or "to see," but has a slight nuance difference. Somehow, 看看 has a more tentative, and arguably more polite tone than 看。 To me, 你看看！（look!) has a slightly more tentative/polite tone than 你看！

because we only use 看 as a transitive verb. Whereas 看看 or 看一看 can be used as either transitive verb or intransitive verb. 我昨天看了一部片子。vt 让我看看。vi 让我看看这部片子。vt

repetition of the verb means quick action

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