Admittedly, the World's Coolest Rain Gauge™ is a little different than what you might be used to. The numbers on the measurement tube are in reverse order. The tube doesn't hold water. When it's empty, you can't see the numbers. What's going on here?

Blame it on Archimedes. The science behind our floating rain gauge™ is based on his eponymous principle of water displacement. As water flows into the copper collection flute, the measurement tube rises to show water accumulation. To paraphrase Archimedes, "Eureka! That's cool!"