Ice hockey is different from other sports in that it does not have boundary lines specifically on the ice that denote inbounds and out-of-bounds. Instead, the rink has boards and safety glass which surround the ice separating inbounds from out-of-bounds.
The puck can go out-of-bounds by flying over the safety glass, causing a brief stoppage of play. When this happens a face-off is used to put the puck back into play. We'll learn more about face-offs in future chapters.
PRO TIP: If the puck gets stuck on the top of the boards, it is still considered in play.