| Why do nebulas like the Crab remain existing for so long? | 1 | | | | |
| Introductory course chemistry | 1 | | | | |
| Can you recommend me a good course/book about the basics of chemistry? | 1 | | | | |
| Using today's technology, would it be possible to design (lethal) booby traps that would stand the test of time? | 1 | | | | |
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