It's a tipping bucket rain gauge. I've posted a picture of what it looks like on the inside and I'll explain how it works. The measuring device actually looks like a see-saw or teeter totter that you would normally find in a park. When one side fills up with water, it dumps through a hole in the bottom of the canister. The other side of the bucket, now in the up position, catches water until it fills up. It will then dump and the process goes back and forth. Generally speaking, each time the bucket dumps, there has been one-hundredth of an inch of rainfall (.01"), but other units can be used. The best part about it; you never have to empty the rain gauge and what dad wouldn't appreciate that.
If I still have your interest at this point, you are probably wondering how much one costs. Well, prices will range depending on where you get it, but the cheapest ones might go for $50, while other models will be slightly over $100. I'm a weather geek and I purchased one many years ago to install on the farm. The best part about it is going to the local COOP and telling the area ranchers and farmers that you had .05" of rain and watching them look at you funny, wondering how you get such an accurate rainfall reading.
To my dad and all the others out there, Happy Father's Day!