The only substance that exists on Earth at standard pressure as a solid, liquid, and gas is water (H₂O).

Here's why:

* Solid: Water exists as ice below 0°C (32°F).

* Liquid: Water exists as a liquid between 0°C and 100°C (32°F and 212°F).

* Gas: Water exists as water vapor (steam) above 100°C (212°F).

This unique property is due to the strong hydrogen bonds between water molecules, which give water a relatively high melting point and boiling point.