Type I superconductors are materials that exhibit superconductivity at very low temperatures. They can completely expel magnetic fields, a phenomenon known as the Meissner effect. This means that when cooled below a certain critical temperature, they allow electrical current to flow without any resistance. These superconductors are typically pure elemental metals, such as lead or mercury. They have a single critical magnetic field, beyond which they lose their superconducting properties. Type I superconductors are simpler in behavior compared to Type II superconductors, which can allow magnetic fields to penetrate in certain regions.