Important Scientific Principles

In high school science classes, I learned some useful principles, usually distilled into pithy little aphorisms.

Biology:
"Xylem, up; phloem, down." and "A dry earthworm is a dead earthworm." These cover both botany and zoology.
Chemistry:
"Ice floats because God loves fish." and "The Laws of Thermodynamics tell us: You can't win, you can't break even, and you can't get out of the game."
Physics:
"F = ma" and "You can't push with a rope."

I also recall that "dirt" is "matter out of place", "noise" is "unwanted sound" and "a weed" is "a plant growing where you don't want it". These tell us that dirt, noise, and weeds are all defined not intrinsically, but situationally, based on human reactions. Soil is not dirt if it's in your garden, but it is dirt if it's on your living room carpet. The sound of flowing water is not noise if you're on a hike near a waterfall, but it is noise if it's coming from your unoccupied bathroom at 3 AM. Dandelions are not weeds if you're growing them to make salad or wine, but they are weeds if you want that green-carpet lawn.
We can reduce the annoyances (like dirt, noise and weeds) in our lives by adjusting our attitudes toward them. There are, for example, few weeds in my life, and relatively little dirt. I will admit that there is a lot of noise, but I realize that's my fault more than anyone else's, and I need to work on that.

What do you need to work on?