Friction comes into action when there is ___________ between the two surfaces

Answer

**Step-by-Step Solution:**

1. **Understanding Friction**: Friction is defined as a force that opposes the relative motion of two surfaces in contact. It is a type of contact force.

2. **Identifying the Requirement for Friction**: For friction to occur, there must be interaction between two surfaces. This interaction is what allows friction to act.

3. **Contact Between Surfaces**: When two surfaces come into contact, they have irregularities or roughness at a microscopic level. These irregularities are crucial for the frictional force to be generated.

4. **Interlocking of Irregularities**: When the two surfaces touch, the irregularities on their surfaces interlock with each other. This interlocking prevents the surfaces from sliding past one another easily.

5. **Conclusion**: Therefore, friction comes into action when there is **contact** between the two surfaces. This contact allows for the interlocking of surface irregularities, which is the fundamental reason for the existence of friction.

**Final Answer**: Friction comes into action when there is **contact** between the two surfaces.

---