re: #14 Gus

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Naturally occurring Radium 226 and Thorium 232.

When my brother was teaching intro Geology classes the one exercise he did every year (or other year) when teaching about radiation and such was issuing Geiger counters and a list of locations and having them find the spot with the highest background radiation on campus. It was the marble facade on one of the main campus buildings.

His other pet one was having them collect water samples from all over to be analyzed for bacterial counts. That always turned up a few interesting surprises.