Q. Why do we dip foods twice on Seder night?
A. During the evening we dip maror (bitter herb) into charoset and karpas (a vegetable such as parsley) into salt water. Some say this is to arouse the curiosity of the participants, particularly children, and motivate them to ask questions.
A Biblical precedent may be closer to the mark, since at the time of the Exodus the Israelites were commanded to dip hyssop into the blood of the paschal lamb; the dipping is thus a symbol of redemption.
The mixture of sour and sweet, bitterness and joy, which recurs throughout the Seder is also illustrated in the two types of dipping: bitter herb into sweet charoset, and karpas (often a relatively sweet vegetable) into salt water.