Knives often come in very handy but rarely has my day been made or ruined by the presence of or lack of one. Occasionally I'll be confronted by a package that defies all attempts to open without a knife, though. A pliers has saved the day occasionally, as has the correct driver bit or screwdriver tip.
_________________________
“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman