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Create a new document. Fill your Background with a color.

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Create a new layer and scribble a few colors onto it.

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Go to the Filter menu and find Liquefy.

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A new interface will pop up. The liquefy tools are on the left side of the screen. You can push things around, twirl them, bloat them, pinch them - play around and see what you prefer. I like to simply use the “warp” tool to create an effect that’s kinda like the wave filter, but with much more control over the direction and movement of the waves.

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Examples:

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Monochrome