Here's why:

* Sedimentary rocks are formed from the accumulation and cementation of sediments. These sediments can be derived from pre-existing igneous, metamorphic, or even other sedimentary rocks.

* Metamorphic rocks are formed when existing igneous, sedimentary, or even other metamorphic rocks are subjected to heat and pressure, causing changes in their mineral composition and texture.

* Igneous rocks are formed from the cooling and solidification of magma or lava.

Melting is the key process. It occurs when the temperature of the rock reaches its melting point. The melting point can be lowered by:

* Increased heat: Heat can come from the Earth's internal heat, volcanic activity, or the intrusion of hot magma into existing rocks.

* Decreased pressure: As rocks are buried deeper, the pressure on them increases. When pressure is released, such as when a rock mass is uplifted or eroded, it can lower the melting point.

* Presence of water: Water can act as a flux, lowering the melting point of rocks.

Once a rock melts, it becomes magma. The magma can then cool and solidify to form new igneous rocks, or it can erupt onto the Earth's surface as lava.