The majority of sponges encountered either in the sea or on the shore are harmless to humans, although a few venomous species exist. These have tiny spicules made of silicon, which can passively penetrate the skin and deliver venom. The effect of picking up a venomous sponge is not immediate, but some time after the contact, the area of skin touched becomes red and blotchy, and swelling can occur. This is accompanied by quite severe pain, which can last, intermittently, for days or even weeks.