Sometimes it can be a tight fit installing a headset cup / race. Bike forums are full of people saying putting the cup in the freezer helps installation based on their anecdotal, uncontrolled experiments. Internet forums are also full of people saying it doesn't help at all, or you need to use dry ice or something else.
Aside from everyone's personal anecdotes, or what someone on the internet said one time, is there any evidence that freezing headset cups makes them easier to install? Does freezing them really shrink them sufficiently to make a difference?