A boulder is a large rock, typically bigger than what one person could lift or move alone. While there's no official size requirement, geologists generally consider a rock a boulder if it's bigger than about ten inches across (roughly the size of a basketball). Boulders can range from that size all the way up to enormous rocks the size of houses or even larger.

You'll find boulders in many places: scattered across mountain slopes, jutting out of hillsides, resting in rivers and streams, or sitting alone in fields. They got there through natural processes over thousands or millions of years. Many boulders were broken off from larger rock formations by the freezing and thawing of water in cracks, which slowly splits rocks apart. Others were carried far from their original locations by glaciers during ice ages, then left behind when the ice melted. Rivers can also tumble and polish boulders as they push them downstream.

Rock climbers seek out boulders for a sport called bouldering, where they climb these large rocks without ropes (though they use thick pads on the ground for safety). Some famous boulders have become landmarks, like Plymouth Rock in Massachusetts or the massive boulders of Joshua Tree National Park in California.