The liquify tool (also spelled Liquefy) is one of the primary ways that people alter bodies with Photoshop.

For this exercise, we'll use a photo of Charles Atlas next to Arnold Schwarzenegger.

Once you open it in Photoshop, click on Filter-->Liquify.

A new window will open with tools on the left and tool settings on the right.  Hover over the tools to see their names.

Try to make Charles Atlas look like Arnold Schwarzenegger!  This requires a good eye; do your best to make it realistic looking.  Remember that you can undo.