During an Ice Age, when the world is colder, more of the world's water freezes
into ice. So less water is in the oceans than before. This makes the level
of the oceans lower, like the water in your bath is lower if you let some
out. Because the ocean level is lower, more of the land sticks out, like
your knees in the bath.
In some places, this makes islands where before the land had been underwater. In other places, this makes a long bridge of land that connects two different continents. During the last Ice Age, about 12000 BC, there was a land bridge between East Asia and North America, leading from Siberia to Alaska, and most archaeologists think that some people and dogs crossed over this land bridge and began to live in North America at that time. These probably weren't the first people to come from Central Asia to North and South America - the first people may have come in an earlier Ice Age, or they may have come by boat.
To find out more about ice ages and land bridges, check out this book from Amazon.com or from your library:
Archaeology for Kids: Uncovering the Mysteries of Our Past, by Richard Panchyk (2001). With twenty-five projects, like counting tree rings, and serializing cars from photographs. Includes a project on soil types.