There is an evolutionary trend in flowering plants for single flowers to cluster into groups, and then for dense groups to take on the appearance of single flowers. (Think of Hydrangea arborescens with larger flowers on the outside of each inflorescence.) One extreme of this trend is seen in Asteraceae, with plants like asters and daisies where the “flower” is actually a cluster of reduced flowers.
Euphorbia is another extreme of flower clustering and reduction. In fact, it’s more extreme than Asteraceae! Each Euphorbia flower seems to consist of an ovary surrounded by stamens, like a normal single flower. However, the “petals,” if present, are bracts (modified leaves). Each stamen is a flower, and so is each ovary. If you take a close look at the stamen, you’ll see that its stalk has a tiny joint, a node, which is where the pedicel (flower stalk) meets the male flower, reduced to a single stamen. It’s easier to tell something weird is happening with the ovary. Not only does it have a joint, but as the fruit matures the flower stalk elongates and grows out of the “flower.” Very strange.