Imagine finishing off a delicious, cold glass of beer...
when you notice that in the bottom of the glass there is a condom.
Yeah, it's not a condom, but it's about that size & shape with the large word Trojan. I couldn't help but think of there being one in the bottom of my drink the whole time. The advert is glued or melted on the bottom of the glass.
{UPDATE} My buddy Shorty pointed out that the bottom of the glass is actually a pig's nose, just like the pigs in the Trojan commercials. When you tip the glass back, it would look like you have a pig's nose from the front. Now that is clever!