Is it or For instance- if somebody makes an odd statement such as just ate a which of the two would grammatically be appropriate response?

The grammatically appropriate response to an odd statement like "just ate a..." would be "For instance".

Here's why:

* "It is" is used to introduce a statement of fact or a description. It doesn't make sense to use it in response to a statement that is incomplete or doesn't provide enough information.

* "For instance" is used to introduce an example or illustration. It's perfect for responding to an incomplete statement by providing more context or asking for clarification.

For example:

Person 1: Just ate a...

Person 2: For instance, what did you eat?

This response encourages Person 1 to complete their thought and provide more information.