A guy riding on a sailboat climbs its mast and drops a coconut.  He watches it accelerate straight down along the mast until it crashes into the deck.
Another guy who is stranded on a nearby island watches from a distance.  He sees the coconut do 2 things at once, it falls down and it falls sideways too (because the boat is moving).  It still lands next to the mast, but it definitely moved sideways as well as down.

The coconut in the story is what physicists call a projectile.  We will study projectiles moving in 2 dimensions.

Well? Do you believe the coconut story?  Every year, at least one student won't believe it.  Why?  Because they also have experience with projectiles, like rifle bullets or bb's.
They believe that a rifle bullet goes along perfectly flat for quite a distance before it falls.
WELL, IF THIS IS TRUE, THEN I WOULD LIKE TO KNOW WHERE THEY BUY THEIR SPECIAL "ANTI-GRAVITY" BULLETS!!!
The truth is that a bullet shot horizontally (not up!) will land at the same time as one that is just dropped from the same height.  REMEMBER THE COCONUT!

Still don't buy it?  Go to this website, and in the little boxes, click "case2" and set the velocity to 30 or so.  Hit enter, then click start.  If this does not convince you, then you probably also believe in those mystical flat-flying bullets.