The energy-mass relation $E = mc^2$ is everywhere in physics.

A Gaussian integral:

It's on the tip of my tongue. — when you almost remember something.

You nailed it. — you did it perfectly.

Let's play it by ear. — decide as we go.

That rings a bell. — sounds familiar.

I'm on the fence. — can't decide.

It's a long shot. — unlikely to succeed.

Cut to the chase. — get to the point.

We're on the same page. — we agree.

It slipped my mind. — I forgot.