Here's why:

* Resistivity: Each material has a property called resistivity (represented by the Greek letter rho, ρ). Resistivity is a measure of how strongly a material opposes the flow of electric current.

* Resistance and Resistivity: Resistance (R) of a material is directly proportional to its resistivity (ρ), length (L), and inversely proportional to its cross-sectional area (A). This is summarized by the formula:

R = ρL/A

Examples:

* Copper: Copper has very low resistivity, making it an excellent conductor of electricity.

* Rubber: Rubber has very high resistivity, making it a good insulator.

* Nichrome: Nichrome (an alloy) has high resistivity and is often used in heating elements.

Key takeaway: The resistance of a material is determined by its inherent ability to resist the flow of electric current, as measured by its resistivity. Different materials have different resistivities, leading to different resistances.