The fruit of the stickseed, shown above, are a marvelous adaptation for mammalian dispersal. The exterior of the fruit is heavily armored with stiff hook-tipped bristles - ideal for adhering to passersby. Many a hiker has had the misfortune of inadvertently bumbling through a patch of this plant, only to look down and see that his/her legs are covered with burs.
Just as surely as tanagers and warblers migrate, so do plants. Plants have all sorts of migratory ploys in their bag of tricks, including wind and water dispersal, transport by all manner of insects, forceful physical ejection of seeds, and more. People probably tend to notice the mammalian dispersal plant tactics the most, because these plants don't discriminate against the various mammal species so we get involved in the process too.
Next summer and fall, when you pause to pluck burs from your clothes and cast them to the ground, just take note that you've been outfoxed by the plant world.