Over time, factors like diet and smoking can change how our DNA is expressed, which is called epigenetics. These environmental factors often cause certain chemicals that are part of the methyl group to attach to the DNA. But the methylation does more than just change how the DNA is expressed—it changes the DNA's melting point. The researchers took cheek swabs of five pairs of identical twins, extracted the DNA, and then identified spots in the DNA with particularly indicative types of methyl chemicals had bonded. The researchers then calculated the melting point of the DNA and found that the methylation changes the melting point ever so slightly, allowing the researchers to distinguish one twin from another.