Human penis
A penis can become erect ("hard") if a  male is sexually aroused (or sometimes during sleep, even though there is no sexual stimulation). In an erection, the penis fills with blood. The blood makes the penis become longer, thicker and harder. Veins taking blood away from the penis get smaller, so less blood is taken through. Arteries bringing blood to the penis get wider, bringing more blood to the penis. Erections sometimes occur in the male fetus (before birth), and occur naturally from birth onwards.  It is normal for the penis to be erect when a male wakes up. This is commonly called morning wood. It is also normal for male teenagers and men to have an erection several times per night when sleeping. If this does not happen, the person may have a bad medical problem or depression (a mental health problem.) Not being able to have an erection can also be caused by various common drugs, including antidepressants and drugs taken to control high blood pressure. During the day, an erection sometimes happens for no reason, or when undressing. This is most common in young teenagers and is normal.  People call this a spontaneous (sudden or surprise) erection. A slang word for an erection is a boner.