Jenny pulled up to her Mom's house and cut the engine. "Oh, Mom! I hope you have something quick for dinner; I'm starved!"

"I'm just going to do some spaghetti with a red sauce; is that OK?"

"Now, Mom! You know I love your pasta however you make it!"

Her Mom unlocked the door and Jenny took in the bags from their shopping trip.

"I can get the water going on the stove. You want me to put the cold stuff in the fridge?"

"Thanks, Honey! I'm going to start on the sauce. You know, I always use the stuff in a glass jar and then mess with it a bit, don't you?"

"Yup, but it's how you do the messing that I need to learn!"

Jenny stood with her hands in her pockets as she watched her Mom struggle with the jar of sauce, tapping the metal lid with the edge of a knife.

"Oh, darn! This jar just doesn't want to open! Jen, can you give it a try for me, please?"

Mom handed the jar to Jenny, who reached out her left hand and took it by the top. She said, "OK, here is where I do my magic trick!"

She then took the jar top by her other hand and held it up, grinning, saying, "Now watch this, Mom! Nothing up my sleeve!"

The jar opened instantly, with hardly any effort!

What trick had Jenny performed?