Why is the onion a but potato not?

The statement that an onion is a "but" and a potato is not is incorrect and doesn't make sense.

Here's why:

* "But" is a conjunction, not a descriptor. It's used to connect contrasting ideas in a sentence.

* Onions and potatoes are both vegetables. They are both edible parts of plants and are commonly used in cooking.

The difference between them:

* Onions belong to the *Allium* genus, which also includes garlic, shallots, and chives. They have a strong, pungent flavor.

* Potatoes are tubers, which are underground stems that store food. They have a starchy and mild flavor.

So, there's no basis for saying an onion is a "but" and a potato isn't. They are both vegetables, but they have different characteristics and are classified differently in the plant kingdom.