Question: what is an erection?

Andrew Cairns answered on 13 Nov 2011:

Scientifically speaking, an erection is when blood rushes to the soft, sponge-like tissue making up the penis, making it hard (erect). This is usually caused by a change in the hormonal balance within the blood. How this effects the size and shape of the penis depends on the species – both vertebrates and invertebrates can have erections. The barnacle has quite an impressive record on this front, as do several other species in the animal kingdom!