Merge Bodies
Create geometry by combining two or more bodies. The merged geometry remains in the body to modify.
1. Under Bodies, make sure the Body to modify collector is active, and select one body to modify. For example, select the blue sphere.
2. Make sure the Modifying bodies collector is active, and select a body or bodies to merge into the body to modify. For example, select the white cube.
3. Click 
OK. The body to modify keeps its blue color. The modifying body is consumed.
OK. The body to modify keeps its blue color. The modifying body is consumed.