This tutorial will teach you an advanced feature of the Liquify filter that allows users to only liquify parts of their image while retaining others.

1. Open up the document you want to add the liquify effect to. For this tutorial, I will be using the image shown below. Feel free to use this image for practice:

2. Go to Filter > Liquify. The liquify tool can be used to distort your images in all sorts of unique ways, similar to the distort or transform tool in Photoshop, except by using a brush instead of points.

3. The picture above is what the Liquify Filter Tool should look like. To begin freezing parts of our image, select the Freeze Mask Tool ( ) From your liquify filter tool bar (located on the left). Select a brush size of about 50 (located on the right), and then start brushing over your image where you want to retain parts of your document. As you brush, the parts being masked will turn red:

4. Now, select your Thaw Mask Tool ( ) and with a fine brush, start removing any parts of your mask that go over the parts you wanted to retain:

5. Time to liquify. Select one of the liquify tools, and begin brushing around, when you apply the filter, you will see that the parts frozen will stay the same!

With this technique, you can make some pretty nice pixel stretch effects, and some great manipulations! Good Luck!