Jonathan Zimmerman explains why teachers should invite, not censor, tough classroom debates.
- Just 100 companies produce 71 percent of the world's greenhouse gases.
- This map lists their names and locations, and their CEOs.
It marks another milestone in SpaceX's long-standing effort to make spaceflight cheaper.
- SpaceX launched Falcon Heavy into space early Tuesday morning.
- A part of its nosecone – known as a fairing – descended back to Earth using special parachutes.
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