Each base on one of the strands binds to one specific base on the second strand. This specific binding is called complementary. Purine bases are complementary to pyrimidine bases (that is, they are capable of forming hydrogen bonds with them): adenine forms two hydrogen bonds only with thymine, and cytosine forms three hydrogen bonds only with guanine. In a double helix, DNA strands are also linked by hydrophobic bonds.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.