Why do a and t (and g and c) pair in a dna double helix? covalent bonds form between them. van der waals forces are thus maximized. ionic interactions are thus maximized. hydrogen bonds form between them.

Hydrogen bonds form between them.

Explanation:

The two strands of DNA bond through the hydrogen bridges between the nitrogenous bases. Adenine forms 2 hydrogen bridges with thymine, while cytosine and guanine form 3 hydrogen bridges. Since hydrogen bonds depend on strict standards of hydrogen bond donors and receptors, and because these structures must be in the right places, hydrogen bonding requires complementary chains of adenine and thymine, and cytosine and guanine.

if you're asking what comes out, it's steam.