Globally, condoms are the most common method of birth control.[50] Male condoms are put on a man's erect penis and physically block ejaculated sperm from entering the body of a sexual partner.[51] Modern condoms are most often made from latex, but some are made from other materials such as polyurethane, or lamb's intestine.[51] Female condoms are also available, most often made of nitrile, latex or polyurethane.[52] Male condoms have the advantage of being inexpensive, easy to use, and have few adverse effects.[53] Making condoms available to teenagers does not appear to affect the age of onset of sexual activity or its frequency.[54] In Japan, about 80% of couples who are using birth control use condoms, while in Germany this number is about 25%,[55] and in the United States it is 18%.[56]