If you can't get a lid off a jar,why does hitting it with something,say a butter knife help?

Often it is just a little of the filling that gets splashed onto the seal and the rap is enought to make the bond break.

Put the edge of the seal under hot water for about 2 mins (this helps it loosen), and play with the butter knife with the jar's edges. If it's still sealed after that; immediately use a hand towel (or your shirt) to open it by twisting (But after the running in hot water). You'll have it pop open in no time.

Try some warm running water first...it may expand the metal lid a bit...or melt some of the material that is causing the lid to stick.

Rapping it with a knife may break the vacuum seal by creating a small leak...or it may crack the glass, so be careful !

Yes that helps me everytime because it pops its seal. There is a airtight seal around the lid and sometimes thats why you cant open it. Hit it a few good times maybe more and the seal will pop.

Hitting it will break the seal and thus make it possible to remove the lid.

um why would you hit a jar with a knife? i know not being able to open a jar is frustrating but isn't that a bit much?

it doesn't. you want to break the seal, so a more intelligent Solution would be to try to pry the lid with a spoon to pop the seal.

eye lids cutt off, but why with a butter knife?

well you have to hit it with a butter knife a certain way ( hit it in a slanted way, the way you would turn it). well it just loosens it up for you.

get a plastic container/surface that will allow you to gather what is in the jar, and then proceed to smashing the jar on this surface/container