The Talmud explains that by beginning the Seder meal in an unusual way, with a vegetable instead of with bread (matza actually), the children will be curious and ask, "Why are we beginning the meal with a vegetable instead of bread?" Once their curiosity is aroused, they will be more attentive to the story of the Exodus. Why a vegetable? Just as a vegetable serves as an appetizer, so too the unusual things we do this evening are meant to whet the children's curiosity.