Today's soldiers frequently utilize high-quality helmets composed of ballistic materials like Kevlar and Twaron, which provide better protection. Some helmets are also effective non-ballistic protectors against hazards like concussive shock waves from explosions. When soldiers went to war in the early 20th century, they often did not have choice in what type of helmet they wore. For example, American troops fighting in World War I were given flannel shirts and trousers, leather boots, and woolen hats; some of them also had steel helmets to cover their ears and head while others didn't. The British used tin cans for helmets during this time period. In the late 1930s, the United States began developing its own style of helmet. This new helmet was made out of a lightweight alloy called "cobalt" instead of metal. It included a thick padding on the inside to absorb the force of impacts and prevent brain injury. The next major advancement in military helmet design came after World War II. The introduction of the plastic helmet in 1951 by the U.S. Army was a great step forward because it provided better protection for soldiers who might be injured or killed by artillery fire. Since then, modern helmets have continued to improve.