How did the giant close his cave in the book of Odysseus?

In Homer's "Odyssey," the giant Cyclops Polyphemus closes the entrance to his cave by rolling a massive stone over it, which is so heavy that no one but he can move it. This stone effectively traps Odysseus and his men inside the cave as Polyphemus tends to his sheep and guards his domain. Odysseus later devises a clever plan to escape from the cave after blinding the Cyclops.