Have you ever wondered what the rules of American Football are? Here's a quick guide.

Rachel's English: For some people, "s" and "sh" sound the same. As Rachel explains, the positions for these two sounds are quite different. Learn the difference in tongue and lip position, and practice with minimal pairs.

There are few places on Earth as breathtakingly spectacular as the Grand Canyon. One of the last great American wildernesses to be charted and explored, this mile deep rift in the landscape has been designated a national park. Among the wildlife to dwell in this remote and often harsh environment are the sure-footed, hard-headed, and sometimes quarrelsome bighorn sheep.

When battle is done, the exhausted bighorns make their way to the canyon rim. But they must be wary, as the canyon is also home to large predators, and a fierce mountain lion is on the prowl. The awe-inspiring vastness of the Grand Canyon attracts numerous tourists, but many will go no further than the lookout points due to the challenging terrain.

The woodpeckers have cleverly used a pine tree to hide the acorns they collect, and are very good at working together. They must simultaneously keep a lookout for predators and guard their wealth from other animals keen to pilfer an easy meal!

One of the rarest birds in the world, and the largest in the Americas, the California condor soars majestically over the canyon. A growing herd of bison roams the north rim, but their presence in the park is ruffling some feathers!

The prairie dogs are on constant alert, but the kit fox has a young family to feed, and prairie dogs are certainly on its menu! As the heat rises and birds circle the sky, the creatures below need to assess which may be a threat, but sometimes, not all is as it seems...

For centuries, the Grand Canyon and its river were thought to be impassable, but in the nineteenth century, steep paths were carved into its walls. However, for the agile bighorn sheep and summer visitors like the violet-green swallow, this is still nature’s playground.

Tourists come to raft the immense Colorado River rapids, but this is no amusement park and there are very real dangers within the canyon’s gorge. Meanwhile, a graceful osprey fishes the turbulent waters, but must also contend with the cunning raven, who is determined to steal his hard-won catch.

People have traveled through the canyon for millennia, and some even made this place their home. However, in the extreme weather of the monsoon season, this is a perilous and inhospitable environment for all but the best adapted native wildlife.

The rare California condor has been successfully re-introduced in the Grand Canyon, and scientists monitor their progress by tagging the individual birds to identify them. As scavengers, they feed on the carcasses of dead animals, but are actually very clean creatures.

Pronghorns are extremely fast and can easily outrun a mountain lion! Even so, they must watch where they tread as the canyon floor is littered with holes... prairie dogs live here and may share their burrows with some unusual birds that nest underground.

A fearsome American badger is on the hunt, and the prairie dogs are not even safe in their burrows. The north rim of the canyon thunders with rutting bison, as the big males lock horns to establish superiority.

With the winter drawing in, the animals prepare once more for that harshest of seasons within the breathtaking confines of the world’s greatest gorge, the Grand Canyon National Park.

Forged by the volcanic heat that still bubbles at the heart of this great American wilderness, the mighty Yellowstone was the first of America’s National Parks. Located mostly in Wyoming, it is a place where bison roam free and the gray wolf has returned, re-establishing the natural order of the ecosystem.