Momentum, a function of mass and velocity, is conserved between portals. In layman's terms, speedy thing goes in, speedy thing comes out.

Look: we're both stuck in this place. I'll use lasers to inscribe a line down the center of the facility, and one half will be where you live, and I'll live in the other half. We won't have to try to kill each other or even talk if we don't feel like it.

This is your fault. I'm going to kill you. And all the cake is gone. You don't even care, do you?

Area and state regulations do not allow the Companion Cube to remain here, alone and companionless.

While it has been a faithful companion, your Companion Cube cannot accompany you through the rest of the test. If it could talk - and the Enrichment Center takes this opportunity to remind you that it cannot - it would tell you to go on without it, because it would rather die in a fire than become a burden to you.