When the Torah commands us to tell the story of the Exodus at the Seder, it frames the entire experience as a parent responding to a curious child’s questions.1 To spark that curiosity, the sages filled the night with unusual rituals that make a child stop and ask, “What’s going on here?”2

One such ritual is the dipping of the karpas.

Think about it: We take a simple vegetable, dip it in salt water, and eat it before the meal even begins. Naturally, the child wonders: “Why is this night different from all other nights?”

Which Vegetable?

The Talmud does not specify which vegetable to use for karpas—any vegetable is acceptable, except those used for maror.

The custom developed to use a vegetable called karpas (כרפס), as its letters can be rearranged to spell ס' פרך—samech (numerical value of 60) and perech (hard labor)—alluding to the 600,000 (60 myriads of) Jews who were enslaved in Egypt.3

Which vegetable is karpas? Some identify it as a leafy herb, like parsley or celery.

Yet, others prefer root vegetables such as potatoes, radishes or onions. Chabad (as well as some others4) use either onions or potatoes.

Why these two?

A practical explanation often given is: That’s what was available.

In Eastern Europe, by the time Passover rolled around, there weren’t many fresh vegetables left. After a long, frigid winter, hearty root vegetables like potatoes and onions were often all that remained.

But that can’t be the whole story. If this were purely about survival, we wouldn’t expect it to become an official, codified Chabad custom. And the fact that onion is actually preferred over potato in Chabad practice suggests there’s something deeper going on.

Let’s explore a couple of ideas that may shed light on this tradition.

A Bit Bitter—But Not Too Bitter

The karpas dipping is meant to hint at the bitterness of slavery, so a bitter-tasting onion fits the bill. In fact, bitter herbs would perhaps have been an even better choice were we not saving them for their special place later on in the Seder proceedings.5

The Blessing on Onions

This one’s a bit more technical—but fascinating.

When it comes to blessings on food, the default blessing is Shehakol (“Blessed are You … by Whose word all things came to be”). The sages also assigned specific blessings to certain categories of food—for example, Ha’etz for fruits that grow on trees and Ha’adamah for vegetables and produce that grow from the ground.

However, these specific blessings only apply when the food is eaten in its usual, culturally “optimal” form. If a vegetable is normally eaten cooked (such as a potato), eating it raw lowers its status to Shehakol. The same is true in reverse: if a food is typically eaten raw (such as lettuce), cooking it would change its blessing to Shehakol.

This brings us to raw onions.

They’re not usually eaten on their own—but they’re often eaten with other foods, like in a sandwich or a salad. So does a raw onion that’s eaten on its own warrant a Shehakol or a Ha’adamah?

Some halachic authorities say Shehakol, since people don’t usually snack on raw onions solo.6

But Rabbi Schneur Zalman of Liadi, founder of Chabad, (as well as others7) disagrees. He rules that since raw onions are regularly eaten in combination with other foods, they still count as a food typically eaten raw—and therefore, the blessing remains Ha’adamah, even when eaten alone.8

(See: What Is the Blessing on Onions?)

Based on this, some suggest that Chabad prefers an onion for karpas to highlight this blessing and show an affinity9 for this ruling.10