Here's why:

* Purines are a type of double-ringed nitrogenous base found in DNA and RNA.

* Adenine (A) and Guanine (G) both have a double-ring structure, making them purines.

* The other two nitrogenous bases, Cytosine (C) and Thymine (T) (or Uracil (U) in RNA), are pyrimidines, which have a single-ring structure.