To check tyre tread properly, use the 20p coin test (or a penny if you're in the US): insert the coin into the main tread groove; if the outer band is visible, the tread is below the 1.6mm legal limit ...

Tires degrade naturally over time and suffer even more damage from potholes, glass, nails or other debris you may run over. If you maintain your tires by keeping them inflated, getting them rotated ...