This achievement is story relevant so it can't be missed.
*** Spoiler - click to reveal ***
In the present, have Bernard go to the second floor and enter Ed's room. If you have angered him
, give him back his stamp collection.
Then pick up his hamster.
On the floor below is an ice machine on the left. Put the hamster in the freezer.
In the future, when Laverne is able to walk around in her tentacle costume, go to the former diner room and put the frozen hamster in the microwave.
Remember that you get
here, too, if you put the hamster in the microwave again.