Okay kiddo, Lewis and Clark were two guys who went on a big adventure a long time ago, back when the United States was really new.
Their adventure started in 1803 when President Thomas Jefferson asked them to explore a big piece of land called the Louisiana Territory. Nobody had really gone to that part of the country before, and President Jefferson wanted to know what it was like.
Lewis and Clark had a whole bunch of other people with them, like soldiers and guides, and they were all in really big boats called canoes.
They started out in Missouri and traveled up the Missouri River, which is a really long and wide river. They stopped at a lot of different places to look at plants and animals, talk to Native American tribes, and make maps.
It took them about a year and a half to get to the Pacific Ocean, which is really big and has a lot of interesting animals, like whales and seals. They went through some really tough places, like over the Rocky Mountains and through really cold winters, but they didn't give up!
After they got to the Pacific Ocean, they turned around and went back to Missouri, through some different paths this time. It took them another year and a half to get back, so the whole trip took about three years.
When they got back, President Jefferson was really happy and proud of them. They had found out a lot of new information about the Louisiana Territory and the animals and plants that lived there.
Today, we still remember Lewis and Clark as brave adventurers who explored some really tough places and helped us learn more about the world we live in.