Cross Sectional Area

[hotcommodity]

So I thought I had a good grasp on rotating functions and evaluating volumes, but apparently not. I'm having trouble picturing the cross sectional area of a 2-d graph. I get that when you have a volume (3-d) of say, a sphere, the cross sectional area is simply a circle, or when you have a cube, the cross sectional area is simply a square. But the 2-d graphs confuse me. Like the bottom two graphs on this page:

 

http://www.pinkmonkey.com/studyguides/subjects/calc/chap8/c0808301.asp

 

Could anyone help me picture why the one graph has a cross sectional area of a circle and why the other has a cross sectional area of an equilateral triangle, thanks...