Water Bird
Description
You’ll see this called “dippy bird”, “drinking bird”, and “bobbing bird”. We’ve all come to recognize it as a silly cheap toy, but did you know that it was patented by an engineer named Miles V. Sullivan in 1946?
The dippy bird is a glass device filled with a solvent with a low boiling point, such as methylene chloride. It has a glass bulb on the bottom, one on the top, and a glass tube connecting the two. The top bulb is covered in felt. When you wet it, as the moisture dissolves in the air, cooling the “head” and changing the pressure inside the bird, which pushes some of the solvent upward, causing it to tip. Gravity will drain the water back, causing the whole thing to bob back to its original position. This cycle will complete until the water is completely evaporated.
People mistakenly call this “perpetual motion”, but of course energy isn’t being created from nothing, and when the water evaporates it all shuts down.
How to get it in ACNH
How to get it IRL
You'll find these all over Amazon at extremely low prices, in many case because it's sold directly by the overseas manufacturers. I would exercise a little caution here: despite product copy saying that the liquid inside is "environmentally friendly" and "no damage to the skin", they're probably lying. You can't achieve this kind of effect without a solvent like methylene chloride, and that is certainly NOT harmless if touched or ingested.
It used to be made by manufacturers like Westminster and TEDCO, but it looks like those companies have stopped producing them—likely due to those very liability issues. It looks like your only option are the cheap Amazon ones, but just be careful. If the package comes damaged, throw it away immediately. And even though they call it a "toy", keep it in a safe place and do not let children play with it unattended. 
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