Butane is ordinarily a gas, but it can be liquefied at room temperature with a pressure of about 2.5 atm (more than double normal atmospheric pressure). When the release button is pressed, the pressurized butane escapes into the air as a gas. The photo on the left shows a "full" lighter. The photo on the right shows the lighter after half of the butane, which is transparent and colorless, has been allowed to escape. I have also removed the label that was obscuring the lighter in the first photo.