The buttercup flower at the right, which showed up as a welcome "weed" in my garden, shows the main features of buttercup flowers.
Mainly, notice that like most (but not all) buttercup flowers it has five separate petals that are yellow (sometimes white). Unlike our Standard Blossom with its five stamens, buttercup flowers have numerous stamens with slender, yellow anthers surrounding the green center. That green center is composed of a cluster of several distinct, unconnected pistils (the flower's female part, as explained on our Standard Blossom page), which will develop into small, dry fruits known as achenes, as described on our Simple Fruits Page.
When the pistils are pollinated, the stamens and petals fall away, as shown in the orange-framed inset at the picture's lower left. The flower shown there has just a few stamens and a single petal still attached. To its right is a fruiting head, with all the stamens, petals and sepals fallen away, and the pistils enlarged into achenes.
The inset at the lower right shows the much enlarged bottom of a petal pulled from the flower in the picture. At the very base of the petal there's a cuplike scale. Between the scale and the petal an insect will find a tiny amount of sweet nectar. This structure with its "secret" little pool of nectar is known as a nectariferous spot or pit. If you find a small yellow blossom and you're not sure it's a buttercup, look for the nectariferous spot at the base of each petal!