The function of adhesives is to hold things together. Adhesives come in many forms, including glue, tape and sealant, and are used in a variety of everyday applications. In addition, natural substances can act as adhesives, such as soda and caramel. The stickiness of the adhesive is caused by molecular bonds, and its strength depends on how much stress is needed to pull those bonds apart. Every substance contains molecules that have a positive or negative charge. For two substances to bond, their opposing charges need to be attracted to each other in the exact amount of charge. For example, to produce a molecule of hydrogen chloride (HCl), the positive charge of the hydrogen atom bonds with the negative charge of the chlorine atom, and the combination of the two creates a strong, neutral bond. Adhesives contain long chains of protein molecules that bond with the molecules of the surface they are spread upon by entering the pores and spaces of the material. Therefore, smooth surfaces such as glass don’t react well to adhesives because there aren’t any pores for molecules to enter. Adhesive strength is the measurement of adhesion, or the attachment between adhesive and substrate. This may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms. The strength of adhesion depends on many factors, including the means by which it occurs.