It's programmerese going way, way back: Alt-Enter in Excel inserts a line-feed character, whuse ASCII value is 10. If you know any character's ASCII value, you can hold down the Alt key, type the ASCII value on the numeric keypad (not the top row above QWERTYUIOP), and when you let go, that character will be inserted. Check out the Character Map (%windir%\system32\charmap.exe): use the arrow keys to navigate, and watch the bottom row.
Here are a few other characters:
Backspace=8=Ctrl-H (Backspace key)
Horizontal tab=9=Ctrl-I (Tab key)
Line Feed=10=Ctrl-J
Vertical tab=11=Ctrl-K
Form feed=12=Ctrl-L
Carriage return=13=Ctrl-M
End of file=26=Ctrl-Z
Esc=27=Ctrl-[
Generally the Enter key inserts both CR and LF on DOS systems, but just LF on *nix systems.