Mnemonic Device:

• Raising Elephants Is So Utterly Boring

Explanation: In Linux, pressing Ctrl+Alt+SysRq (Print Screen, on modern keyboards), releasing SysRq, and, still holding Alt, pressing R E I S U B in that order (with a slight pause between each) will reboot even the most stubbornly frozen system.

This works because R E I S U B are the magic SysRq keys that correspond to: resetting the keyboard state, sending SIGTERM to all processes, sending SIGKILL to all running processes, synchronizing all filesystems, unmounting all file systems, and rebooting.