Bur medick

A small, sprawling weed. With such clover-like, trifoliate leaves -- leaves divided into three leaflets -- you can guess that this is a member of the Bean Family, which it is. 
This is one of several species of the genus Medicago, commonly known as medicks, though the very important forage plant Alfalfa also is a Medicago, M. asiatica. Medicago species are generally recognized as small, weedy herbs looking more or less. Fruits of most species are smooth, shaped like flattened cloth coin-purses, or sometimes coiled like snail shells.