Why not talk while eating food?

When you talk with food in your mouth, it means you don't have manners.

Edit: Also, bits of food can fly out.
The short answer is : the epiglottis is a movable flap of tissue attached behind/under the tongue which blocks the entrance to the trachea (windpipe) when food is swallowed, thus protecting the lungs by forcing the food to pass over the trachea and instead go down the esophagus (food- & drink-pipe) into the stomach. If you talk while eating, and you happen to be a toddler, your Mum will tell you off. If you talk while eating, and you are older than that, people may think you are a bit of a pig.......