If I half fill a jar with hot water and close it and leave it for a short time, not much happens when I open the jar.  But if I close the jar and shake it vigorously, when I open the jar there is a lot of pressure built up in there.  Why does shaking the jar do this?

It's general knowledge that a salt and ice mixture "lowers the temperature" but let's be more precise. Does mixing salt with ice actually lower the temperature of the mixture or does the addition of the salt simply lower the freezing point of the water, thereby causing the ice to melt into water without actually influencing the temperature of the mixture?

Or does it do both?