Video narration
The shortcut is 2 times the quantity length times width, plus length times height, plus width times height. These are square units because you measure the outside covering, not the space inside. Space inside is volume, measured in cubic units. Now, does every prism look like a box? No. A prism is defined by two matching, parallel bases, with side faces connecting them. Those bases can have shapes other than rectangles, so a rectangular prism is only one kind of prism. A pyramid is different: it has one base, and its side faces meet at one point, called an apex. That is the key flip. A prism has two matching parallel bases; a pyramid narrows from one base to a point. So back to your box: covering it means adding all six outside faces to find its surface area. Two matching parallel bases means prism; one base and a point means pyramid.