Thunder clouds are so impressive. Lightning forms due to air currents in the cloud as warm moist air rises up quickly. All the collisions between particles cause different charges to develop between the top and the bottom of the cloud. When that gets large enough, kapow!

Also see: thunderclap or rumble, dirty thunderstorms.

Thunder clouds

Reproduction. Excess. Variation. Selection. Divergence.

Darwin’s 5 principles of Natural Selection

Very easy to forget the difference between the mean, the median and the mode.

Median: the middle value of a list in ranked order.

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Measures of central tendency: mean, median, mode