【英语时差8,16】南北两极-上（在线收听）

 Say you’re a polar explorer and you’ve just reached the north pole. The huskies are yapping, the cameras are ready and you’re just about to unfurl the flag when, all of a sudden, the north pole becomes the south pole. Imagine your chagrin. Okay, so it doesn’t really happen that fast, but it’s true that in the past the earth’s magnetic poles have switched places. In fact, it seems to happen roughly every 250,000 years or so. How do we know this? When lava is in its molten state, the atoms in it can move around fairly easily. At this time some atoms, like iron, will align themselves with the earth’s magnetic field in a recognizable way. Then, as the lava solidifies into rock, these little compasses get frozen in position. It’s like writing your name in wet cement.