ICSE Class 9 Answered

When two bodies are kept one over the other, then they are not really in contact at all points i.e., real area of contact is much smaller than the total surface area.  At actual points of contact, the distance between the two bodies becomes extremely small and molecular forces come into play. Molecular bonds are formed.  These molecular bonds oppose the motion of one body over the other and are a cause of friction. When one body moves over the other, the molecular bonds have to be broken and so force has to be applied against the frictional force.