Translations:FAQ/56/en

The theory is simple: Lines (or wires), when extruded, form faces. Faces, when extruded, form solids. If you extrude something and the result is not a solid, then the something was not a face. If you have lines and you want to extrude a solid from them, you must first select lines that form a closed perimeter (select several objects by pressing CTRL), join them into a wire (Upgrade tool), then make a face from that wire (Upgrade tool again). There you are, if all went well you can now extrude it to a solid.