When a body is heated, its particles (atoms or molecules) gain kinetic energy, which causes them to move more rapidly and with greater amplitude. This increased motion results in the expansion of the material. The expansion occurs because the particles push against each other and against the boundaries of the material, causing the material to occupy a larger volume.

However, it's important to note that while most materials expand when heated, there are exceptions. Some materials, especially those with anomalous properties, may contract under certain conditions. For instance:

1. Water: Water exhibits anomalous behavior. When water is heated from 0°C to 4°C, it actually contracts instead of expanding. This is due to the unique arrangement of water molecules in the liquid state and the formation of hydrogen bonds.

2. Some alloys: Certain alloys, such as Invar, exhibit very low coefficients of thermal expansion. Invar, for example, is an alloy of iron and nickel with a very low coefficient of thermal expansion, which means it expands or contracts very little with changes in temperature. This property makes it useful in applications where dimensional stability is critical.

3. Certain materials under specific conditions: Depending on the structure and properties of a material, it's possible to induce contraction under specific conditions such as extreme temperatures, pressure changes, or changes in the surrounding environment.