Why does unemployment not go to zero during booms?

What will be an ideal response?

While unemployment has a cyclical component—that is, it naturally goes up during recessions as people are laid off and down during booms as people are hired—there are other components of unemployment that do not vary over the business cycle. For instance, some people are going to be transitioning between jobs at all times of the business cycle (this is frictional unemployment). Frictional and structural unemployment rates are nonzero, so "full employment" actually means that some people are unemployed all the time.