When Pedro Ferreira wants to create a 3d Solid of an area in a drawing (for example the walls of an house), he uses the simultaneous keystroke of CTRL+ALT and picks the area he wants to create into a 3d Solid. This area can be defined by Lines, polylines, regions, etc. Press CTRL+ALT and move the cursor to the area that needs to be a solid. AutoCAD shows where you can create a 3dsolid!
Notes from Cadalyst Tip Patrol: This is a great tip. In reality, this invokes the PRESS/PULL command. But this may be a better shortcut for many users. Press and hold the keys, CTRL and ALT simultaneously. While holding those two keys, move the cursor over a closed area (any combination of lines, arcs, polylines, etc.) similar to creating a hatch, then LEFT-CLICK. This starts the command. Now drag the mouse back and forth to tell AutoCAD where the opposite face of the solid is to be. Using the PRESS/PULL command works the same way. Start the command, select the area (or face on an object) left click, and drag the mouse. This is a fantastic shortcut! Thanks Pedro.