Glues, such as paste or cement, are characterized by joining things together. These materials are known as adhesives because they make things stick together. However, their stickiness is not all they have.
For an adhesive to be practical, it must transform from a smooth liquid to a hard solid in a short period of time. Sticky substances such as honey, syrup, and chewing gum are not good adhesives because they take a long time to harden.
If we were to study any solid body under the microscope, we would realize that what we believe to be perfectly smooth has roughnesses and spaces.
Suppose we are making a model that requires the union of two pieces of wood. We have to apply the adhesive to one of the surfaces and place the other piece of wood on top of it.
What the glue does is cover all those little roughnesses and holes in the wood. When it dries and hardens, the two surfaces are firmly together.
Some adhesives are made from the skin, bones, and tissues of animals and fish!
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