What is the main method of joining materials using adhesives?

The primary method of joining materials using adhesives involves the bonding of surfaces through chemical reactions. Adhesives work by creating a molecular bond between the two surfaces to be joined. This bonding process can be facilitated by various factors such as surface preparation, the type of adhesive used, and environmental conditions. The adhesive penetrates the surface irregularities and once cured, forms a bond that is often stronger than the materials themselves.

In contrast, melting two materials together refers to welding or heat fusion, which is not applicable to adhesives. Performance under high pressure may relate to the bonding effectiveness of certain adhesives, but it is not the method by which they function. Drilling holes for mechanical fasteners involves a completely different joining method that uses physical mechanisms to hold materials together, rather than chemical bonding. Therefore, the correct answer accurately describes the fundamental principle behind adhesive bonding.