The Rosh Hashana custom mentioned in the Gemara and Shulchan Aruch to eat various foods and offer a small prayer with them is quite well known. Apples dipped in honey is probably the most famous, and is meant to enable us to pray for a sweet new year. Someone asked, “Why the apple? Because Rosh Hashanah is in the fall, and apples are in season.” It sounds humorous, but it’s an important concept as well. Apples are in season. We are happy with the things we have, and ask G-d to enable us to taste the sweetness of what He has given us without looking for more.
The Gemara lists certain items to eat, and the machzor (holiday prayer book) has many more items. People constantly invent their own, such as eating peas for Peace on Earth, or eat meat (basar) so that we might have besuros tovos, good tidings. The main thing, says the Mishna Berura, is not to get angry on Rosh Hashanah (not a good idea any time) and to enter the new year with a happy heart, and trust in G-d. It would appear from the Mishna Berura that it’s not eating the food that brings the blessing, but the attitude behind it. So, if someone tips over your raisins and celery, relax, have a good laugh and enjoy a sweet new year.