Glass is blown using fire and water; heating and cooling; insulation and conduction. Hot glass moves while cold glass does not. Trapping air inside a hot piece of glass causes the air to expand, blowing out the bubble. Cooling one section of a glass bubble will restrict that section from being blown thin. Thicker glass retains heat better than thinner glass, though thinner glass heats up faster. Taking advantage of these temperature differences can make blowing glass easy, while uneven, uncentered, unregular things within the glass can make blowing very difficult.