Miley Cyrus celebrated her 18th birthday by experimenting with a bong and catching a case of the giggles -- but sources say she was not smoking marijuana.

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The video was shot during a party at Miley's L.A. area home 5 days after her 18th birthday.

According to a source connected with Miley ... the smoke filling the bong is a natural herb called salvia which has psychedelic qualities. Possession of salvia is legal in California.