Step 1 - Pick up the Arrow tool and select the half of the object you separated in the previous tutorial (A).

Step 2 - Choose Modify>Group (from modify on the menu bar) When you do this a blue rectangle appears around the object indicating that this is a grouped object (B).

Step 3 - Drag the grouped object over another object (C).

Step 4 - Deselect it by clicking on a blank section of the stage.

Step 5 - Click and drag the first object off the second one (D).

Step 6 - With the first object still selected choose Modify>Ungroup Now drag the first object back on top of the second one. Click to deselect (E).

Step 7 - Select it again, and now drag it back off... notice how you're now missing part of the second object? (F).

Grouping objects is an easy way of protecting them from unwanted editing.