Okay, imagine you're going on a really big adventure with a bunch of grown-ups, like your parents or teachers. And this adventure is going to take you to a faraway place called the Mekong River, which is in Southeast Asia. The Mekong River is very long and twists and turns like a big snake, and there are lots of different types of animals and plants living there.
But this adventure is not just for fun - the grown-ups are actually going on a mission to learn about the Mekong River and everything that lives there. They have special tools and equipment to catch fish, look at the rocks and soil, and draw pictures of plants and animals. They want to know what kinds of things are important to the people who live near the river, and how the river helps them grow food and travel around.
The grown-ups are also going to meet lots of different people from different countries who live near the Mekong River, like Laos, Thailand, and Vietnam. They want to learn about their customs and traditions, and see how they use the river in their daily lives.
So for two years, these grown-ups traveled up and down the Mekong River in boats, camping out when they needed to rest. They were brave, because sometimes the river was very rough and they had to be careful not to tip over. But they were also very smart and learned lots of things that they could teach other people back home.
And that's what the Mekong expedition of 1866-1868 was all about - a big adventure to learn about a very important river!