Afflatus

Huginn: I am thinking of expansion.

Muninn: Yes. Very clever. Think big thoughts, why don't you?

H: The ideal gas is one with no particular size, it says here. Its effects are tangible, but it interacts with nothing.

M: I once knew a brain like that.

H: It has mass, but no volume. And its impacts are totally elastic.

M: Yes, it must be the same brain I once knew.