What does Sally sell on the seashore regardless of it’s lack of rarity?

Riddle me this: a ringtone for Archimedes' heart?

I stand in a wall, yet I’m never quite still, I let in the sun, yet I bend to your will. I frame the world, though I hold no brush, I can open to chatter, or silently hush. What am I?

I gloop when you are quiet, I set when you thwack. I ooze from your palm, I punch you right back. I am easy to make, I am half of two to make one. I am hard when you go fast; if I am soft, you will fall. I bend all the rules if you look how I look.

The answer to this riddle is a piece of cake.