My pet light bulb is a year old today

My pet light bulb is a year old today.
That's 5.9 trillion miles in light years.
Your mileage may vary.

This is a compound joke told by Heywood Banks. Apparently it is really hard to understand, so I am going to explain it here. Of the ten people that I told it to today, not one of them understood it.

To understand the first joke, you have to understand the idea of "dog years". Because dogs die at a younger age than humans do, people like to multiply their age by 7, and then say that they are that age in "dog years".

A "light year" is a measurement of distance, though it sounds like it could be a measurement of time (like a dog year.) The first joke is that the joke teller is using it as such.

The second joke is the caveat "Your mileage my vary." In car ads, they often give statistics about how many miles a car will be able to drive for each gallon of gas the car is supplied with. In these ads, invariably, they add the caveat "Your mileage my vary" to protect themselves from legal trouble.

In Einsteinean relativity, we understand that the only thing at all that is not relative is the speed of light in a vacuum, so a light year is something that would never vary at all under any circumstances. The your mileage may vary quote applied to light years is a physics joke because it is patiently absurd.

Finally, the idea of having a pet light bulb is pretty absurd in and of itself.