Jars - too tough and some of the lids are too big for me to get my hand round (that's my pickled onion addiction)
Plastic vaccuum packs - nie on impossible without a chainsaw
Cardboard boxes - have all sorts of random tabs and flaps that you just have to rip off thus ruining the box for future use.
AND NOW...
even bananas are getting in on the act. I've had 3 this week that would refuse to open without squishing the top of the 'nana.
oh and satsumas....