In tennis, why does topspin serve bounce higher than flat serve? When receiving servers (while playing tennis), I've noticed that the tennis ball seems to bounce up higher on me when the server uses a topspin serve than when the server hits a flat serve.  Why is that?
Is it because of something about how the spin affects the bounce of the ball (i.e., the effect of the spin during the time when the tennis ball is in contact with the court)?  Is it because of how the spin affects the flight of the ball while it is in the air?  Perhaps it's because topspin serves tend to be hit with a higher, "loopier" trajectory, and this changes the angle of incidence when the ball hits the court during the bounce?  Or maybe it's something else entirely?  What would you expect the effect of the topspin to be on the trajectory of the ball, based upon physical principles?

Background on tennis: When serving, the server has to hit the tennis ball over the net and then into the service box; the ball is required to bounce within the service box.  The receiver stands back a bit, and after it bounces up, the receiver hits it back.
A flat serve is a serve where the ball has no spin.  A topspin serve is a serve with topspin: the top of the tennis ball is rotating forward (in the direction of the ball's travel) and the bottom of the ball is moving backward (opposite the direction of the ball's travel).  Topspin causes the tennis ball to "curve downwards", due to the Magnus effect.
From the server's perspective, topspin is useful, because it means the server can hit the ball higher over the net and the serve will still go in.  That allows a topspin serve to be more reliable: the server has more margin for error.  In contrast, with a flat serve, the serve has to be aimed very precisely: aim too high, and the ball will "go long" and bounce outside of the service box; aim too low, and the ball will hit the net.  A flat serve will usually look like a "line drive" (the path of the ball is close to a straight line), while a top spin serve will look more "loopy" (the ball goes higher over the net, then swerves down).
Flat serves usually travel faster.  With a topspin serves, the ball usually doesn't have quite as much velocity (I imagine some of the energy of the server is spent on imparting spin, rather than propelling the ball forward).  Nonetheless, from my personal experience, it feels like when I am receiving a topspin serve, the ball bounces up higher and often gets out of my comfort zone: the flat serve might bounce up to waist level, where it's comfortable to hit the ball, but the topspin serve might bounce up to shoulder level, where it's less comfortable.  Why does this happen?  Or, is it an illusion?
 A: When the player hits the ball with top spin, it makes the ball, well, spin. 
By spinning, the ball will modify the airflow around itself and thus create an air pressure profile which will deflect the ball : this is the Magnus effect.
So by applying top spin on the ball the way tennis players do, the ball is rotating in the direction of the trajectory. This will bend the trajectory downwards. If you look at the ball's speed as a vector, the vertical component of the top-spun ball's velocity is greater than the normal served ball.
You simply direct the ball more vertically into the ground with a top spin. So after contact with the ground, the ball with top spin will leave the ground more vertically than a normal ball.
Since you are familiar with the sport, you might also have noticed that after contact, the top-spun ball will slightly accelerate towards you. This comes from the fact that part of its rotation has transfered itself into horizontal momentum. If you let a spinning ball fall vertically on the ground, after contact, it will fly out flat in some direction. I think this is also one of the reason why top spin serves are so hard to handle.
A: Serves hit with topspin will typically bounce higher than flatter serves because:


*

*They are to be hit with a loopier trajectory, to increase net clearance. The apex of a topspin serve will be higher than a flatter serve to begin with. Even if the ball had no spin at all, it would bounce higher.

*The Magnus effect generated by the spin of the ball produces a downward force on the ball, pulling it back down into the court. This increases the vertical velocity of the ball, which will result in it bouncing higher.

*A topspun shot will typically also lose less horizontal velocity due to friction from the ground than a flat shot. In fact, if the tangential velocity of the ball is greater than its horizontal velocity, the force of friction will accelerate the ball forward, resulting in a more pronounced "kick"
An effective topspin serve will bounce above shoulder level for a receiver standing on the baseline, making it more uncomfortable for them to return the ball. That's just biomechanics. 
A: Because you typically also hit topspin shots with more momentum because you can. Without topspin, adding pace to most shots will send them out of the court. 
