It usually happens when you're up to your elbows fixing lunch for a roomful of starving, clamoring kids. (Even if it's just one, it seems like a roomful.) The plastic lid on the peanut-butter jar won't unscrew. Will. Not. Budge. It isn't simply stuck, either. Whoever opened it last put the lid back on crooked, so it's jumped the threads and seems permanently jammed. Before you lose your cool and throw the jar at the guy who screwed it on wrong and tightened it down that way, relax. You can get it off yourself. Easy.
Reseat the Lid
Place the jar or bottle upside down on a firm surface and apply steady pressure, concentrating on the higher edge. Or give the higher edge a firm tap against the same firm surface.
Tap the high edge of the lid gently, but firmly, with a hammer or the bottom of a heavy jar.
Use the tip of a butter knife, the tines of a fork or the pointed tip of a church-key can opener to pry the lower edge of the lid upward.
Wear rubber dishwashing gloves to get a strong, nonslip grip and to protect your hands from the sharp ridges on the sides of many plastic lids.
Create traction by stretching one or two wide rubber bands, such as you find on fresh produce at the grocery, around the side of the lid and around the container.
Dampen a clean (not soapy) sponge and use it to get a strong—but cushioned—grip on the lid
Heat It Up
Hold only the lid or cap (not the jar or bottle) under hot running water for 30 seconds to a minute. Heat will cause the rigid plastic of the lid to expand.
Put about ½ inch of very hot water in a bowl or saucepan and invert the jar in it. Let it stand for a minute to heat the lid.
Try to unscrew the lid or cap immediately after heating it; or you can try again to reseat it properly on the threads, and then remove it.
Use a nutcracker or pliers to grip and leverage smaller lids and bottle caps.
Try a vise grip, locking wrench or pipe wrench for larger lids.
Clamp the lid of a very wide jar in a table or bench vise, to immobilize it while you turn the jar.