It's impolite and unclean. If you truly want to try someone else's food, ask if you can try it before the other person starts eating, and then wait for that person to remove a part from his or her plate and transfer it to your plate or a clean plate brought by the waiter. This way, you are being respectful to their food and they do not feel offended by you getting up from the table. If you are hungry but don't want to offend anyone, you can also use the tip of your knife to dig into the side of the dish you want to sample. You will be able to tell whether or not it is tasty this way before you commit to eating it all. Be careful not to cut yourself or touch anything dirty though! Finally, you can always say excuse me as you get up from the table or leave the room. Most people will understand if you need to go to the bathroom or have an urgent business call to make. But if you stand up without saying excuse me and walk out of the restaurant, that would be considered a rude action too! Getting up from the table when someone is eating is a sign of respect. So before you get up, please ask if it's okay first, so we don't have any problems with unsolicited guests at the dinner table.