Other than on ground balls in the infield, four outs can also come into play when there is a runner on base and the batter hits a fly ball. So let's take a look at how that works and that will also lead us into a topic called the sacrifice fly. Say we have a runner on second base. His goal is to get the third base and eventually to home. If the next batter were to come up to the plate and single, then our runner could jog over to third, even if the batter grounds out, the runner can still make it to third base. But is the runner allowed to move to third base on a fly ball that is caught for an out? Technically he can, but first he must tag up. So let's take a look at what that means. When a batter hits a fly ball that is caught for an out, a force out is created for each runner on the base that they started at. This means that if that runner who was on second base was running toward third base when the fly ball was caught, the defense can get the ball and step on second base to put the runner out. The way to remove this force out is for the runner to return to the base that they started on after the ball was caught. As soon as they touched the base, they started on though, the force out is removed and they are free to run to the next base. This is a risk though because the outbuilder can still throw the ball back in and an infielder can tag the runner out before he gets to the next base. In this context, retouching the base that you started on after the ball is caught is called tagging up. So what will usually happen on a fly like this is that the runner will anticipate that the ball will be caught and so he will wait on the base. And the split second that the right filter catches the ball, he will start running to third base. Again though, he has to be careful. If the right filter has a strong arm, he can catch the ball and throw it to the third base so that he can tag the runner out before he gets there. Most fly balls will not result in a runner tagging up and moving to the next base. Usually the runner will just stay on the base that they started on. In order for a runner to tag up, the fly ball will have to be pretty deep into the outfield. And usually the runner will be tagging up from second base and running to third or tagging up on third base and going to home plate. You almost never see a runner tag up on first base and run to second base. Speaking of tagging up from third base and running home brings us to the next topic, the sacrifice fly. A sacrifice fly or a sack fly, as you might hear called, is what happens when a batter hits a fly ball into the outfield that allows a player to tag up and then run home to score. 99% of the time that runner will be on third base but there is nothing really to prevent a runner on second base from tagging up, running to third and then straight through without stopping to home plate. Something really odd would have to happen for that to take place though, like the ball being hit very deep and probably into foul territory, which I should also mention that runners are allowed to tag up on any foul ball that is caught for an out as well. The other thing to keep in mind about sacrifice flies is that a run must score on the play. So if there is a runner on second base who tags up on a fly ball and runs to third base, it would not be considered a sacrifice fly. He would simply be advancing on the out. So hopefully that helped make a little more sense out of tagging up and sacrifice flies.