Black kidney beans will float after soaking. According to the relevant data, black-flowered kidney beans float because some of them have low density and can float on the water. Dense kidney beans will settle on the bottom of the water. Kidney bean is an annual twining or nearly erect herb in Leguminosae. Stems pubescent or glabrous when old. Feather compound leaves. Stipules lanceolate, leaflets broadly ovate or ovoid-rhombic, lateral oblique, racemes shorter than leaves, and several flowers at the top of inflorescence. Bracteoles are ovoid with several raised veins, calyx cup-shaped, corolla white, yellow, purple or red. Wing flap obovate, style compressed. The pod is banded, slightly curved, the seeds are oval or kidney-shaped, white, brown, blue or spotted, and the hilum is usually white. Bloom in spring and summer.