Why did Liesel deliberately injure her leg while playing outdoors?

Nazi Party members go door to door inspecting basements to identify possible air raid shelter locations. Liesel, who is out on the street playing soccer, sees them and wonders how to go home and tell Hans without seeming suspicious. Liesel accidentally collides with another boy, and Rudy runs to get Hans, who carries Liesel home. Before Hans and Rosa have a chance to figure out how to hide Max, who has since been hidden again in the basement, the party members arrive.