The Crown of Thorns plant is aptly named - although it was not the plant purportedly used as the crown in the crucifixion, it does have red flowers which look like drops of blood.
Although this plant, Euphorbia millii, has leaves topping the thorny stem, this doesn't mean it likes more water. They are so drought tolerant that many novice growers give it way too much water, and kill it off.
One of the give-away characteristics of every member of the genus is white latex-like sap, and this plant is no exception.
Keep in mind that this is caustic and poisonous, especially if it gets on the skin or in the eyes.
Cats in particular like to chew on the leaves. If your kitty seems to do this, keep your Crown of Thorns plant in a bird cage to protect it.
In time, the stems get very woody and tough, and covered in prickles and thorns.
After a period of drought (or even outright neglect) a few weeks of watering regularly will trigger new leaves to emerge from the top of the stems, along with flowers.
The flowers are generally red, or pink, but occasionally they are also white. They are simple, with only two fused petals, and the center of each flower is usually yellow.
Like most Euphorbia, this center part is the actual flower. The brightly colored parts are actually bracts, not petals. Their function is to attract insects who locate flowers by sight. They have little to no scent all all.
Once they finish blooming, prune the prickly stems back by half, to encourage a more compact growth habit.
If you don't do this, be prepared for the plant to make one or two long gangly stems.
Plant in very well drained soil, such as cactus soil. They don't like having wet feet.
Water them in the sink and allow the pot to drain - don't use a saucer under the pot unless you check it often to make sure no water has accumulated.