This is Scientific American's 60-second Science, I'm Christopher Intagliata.If you find yourself at a cookout this 4th of July, gazing at the hot dogs, wondering ... what exactly are they?Here's your answer:"They're just tubes of fat."Tyler Rouse, a pathologist at the Stratford General Hospital in Ontario, Canada.And I may as well give a spoiler alert before continuing this story,because "I've ruined hot dogs for many people."A couple years back, Rouse, too, was wondering about the composition of franks.Given his day job, "I said, that's an easy answer to find out. We work in a lab, we make slides all day.Hot dogs are kind of the perfect shape to make into a slide. We can actually answer this question."So he and his colleague Jordan Radigan got their hands on three types of dogs:a no-name brand from the supermarket, another all-beef dog and a third from a ballpark vendor.They then took cross sections for slides and used stains to identify different types of tissue.And found, to their surprise, that most slices consisted primarily of fat globules,with very little skeletal muscle―the stuff we tend to think of as "meat."In fact, the no-name brand actually had more skeletal muscle than the all-beef brand.The researchers also found bits of bone and blood vessels and cartilage―even plant material.How did vegetable matter get in there?"Let me put it this way. Sometimes I get biopsies from human colons and I find vegetable matter. I'll just leave it at that."