Fake Beer creation for glass? Help!

I'm a product photographer for an online website who has a wide variety of drink-ware.  Most of the time I'm building sets, making things out of Styrofoam, etc. but over the past couple of years, I've purchased quite a few fake pint glasses and beer mugs from an online fake food company.  The product is so perfect and realistic, I want to learn how to make them myself.  For one, I really enjoy making props, and for two, we have a large amount of products that it would probably save me a lot of money.

I have a great appreciation for the fake food company's work, and I don't have a huge problem paying them every time I need something, but sometimes they cannot ship to me in colder months due to the expansion of the glass in colder temperatures.

My question is, how do they do it?  The material is as hard as a rock, and has been poured into real glass.  It captured the bubbles and truly looks like the glassware is filled with beer.  Maybe some kind of deep pour resin, or maybe even UV cured to capture the bubbles?

Any suggestions would be greatly appreciated.  Thanks.

- BaldPhotoFella