Describe what happens between Beowulf and Grendel?

When Grendel enters Heorot, Beowulf is waiting for him. As Grendel goes to grab him, Beowulf seizes his arm and hangs on with all his might. Grendel panics and pulls away, struggling to escape Beowulf's grasp. When he finally breaks free.... he runs from the mead house, leaving his arm behind.