There are numerous lakes and streams that kids can enjoy, the largest being Yellowstone Lake. Most visitors, except those during the warmest days of July and August, forego swimming due to the extremely cold water. Streams and lakes seldom have an opportunity to warm up in this climate, where nights can be fairly cool, if not cold even in the summer months. Without acclimating yourself to these waters, a quick dip can immediately vanish any breath you may have hoped to retain in your body. The two exceptions are Boiling River and Firehole River.