Hyperboloids can be made from straight tilted lines: http://upload.wikimedia.org/wikipedia/commons/a/ad/Ruled_hyperboloid.jpg http://upload.wikimedia.org/wikipe

Hyperboloids can be made from straight tilted lines: http://upload.wikimedia.org/wikipedia/commons/a/ad/Ruled_hyperboloid.jpg http://upload.wikimedia.org/wikipedia/commons/a/ad/Ruled_hyp... So tipi's are hyperboloids with a small inner cross section.