There are plenty of things that are unknown, but are assumed reasonably to exist, even in the most basic sciences. Maybe 90 percent of the mass-energy in the universe is called "dark," because nobody knows what it is.
Dark Energy Mass Month Calendar with this Quote Printable Folded Card Cursive Handwriting Worksheets Quote of the day Bulletin Board Posters
There's lots of good fish in the sea...Maybe...But the vast masses seem to be mackerel or herring, and if you're not mackerel or herring yourself, you are likely to find very few good fish in the sea.
Sea Mackerel Mass Month Calendar with this Quote Printable Folded Card Cursive Handwriting Worksheets Quote of the day Bulletin Board Posters
It was equally imperative that this chain of reactions should always tend to dampen, to die out. it must not build up, or the uranium mass would explode within a time interval too short to be measured by any means whatsoever. nor would there be anyone left to measure it.
Mean Too Short Mass Month Calendar with this Quote Printable Folded Card Cursive Handwriting Worksheets Quote of the day Bulletin Board Posters