They defend an area against rivals of the same species, often so as to sequester a private feeding ground, or a private courtship bower or nesting area.

• 美国国家宇航总局在设计登月车时，曾经花很大力气研究克服月球表面的垂直障碍问题。一般说来，轮子要爬上高度与轮直径相等的垂直障碍是几乎不可能的。