Tear-jerker

A 'tear-jerker' is usually movie or tv show that causes you to suddenly cry.

Example:
-Have you seen the new Meryl Streep movie?
-Not yet, but I heard it's a real tear-jerker!

Knee-jerk reaction

A 'knee-jerk reaction' is the reaction you have to something without giving it much or any thought.

Example:
-Why did you swerve into the other lane of traffic?
-I had a knee-jerk reaction. I saw the deer and didn't want to hit it!