Why are anchors so heavy?
For grown-ups: the science
Anchors are heavy so they can stay on the seafloor and help hold a boat still.
An anchor's holding power comes mostly from its shape digging into or gripping the seabed, plus the pull direction created by its chain or rode. Weight helps the anchor sink, keeps the chain low, and can add resistance, but weight alone is not the main source of holding power.
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Transcript
A big boat bobs on the water. Why does its anchor need to be so heavy?
An anchor drops down to the seafloor on a strong chain. Its weight helps it stay down instead of floating away.
The anchor has wide arms that dig into sand or mud. The seafloor pushes back and helps hold the boat in place.
Wind, waves, and water can pull a boat along. A heavy anchor and chain make that pull much harder to move.
So anchors are heavy to stay on the bottom and help grip the seafloor. That keeps boats from drifting away.