Prank on my dad... (help?)

[FRAG_MASTER]

Hey guys, im gunna try to play a prank on my dad, does anyone with knowlage on writing Batch file know how to write a .BAT file to change the HOSTS file, but in the back ground, so when he opens Firefox, for example, it will change the HOSTS file so he cant open certain websites?

Thanks in advance!!

~FRAG_MASTER

P.S. I would name the .BAT file 'Firefox' and change the icon to make it look like it, not put the .BAT file in the actual Firefox .EXE file.