Spinthariscope
Spinthariscope
[Cueball is holding a small item up in on hand in front of his three friends. Megan has her arms lifted and bent in front of her, White Hat has his arms raised over his head and Ponytail is pointing at Cueball while her other hand, held down behind her, is balled into a fist.] Cueball: It's a spinthariscope, a 1940s toy with a radioactive isotope inside. If you let your eyes adjust to total darkness and look into the lens, you can see the flashes of individual atoms decaying. Megan: What?? White Hat: Aaaaa! Ponytail: Get it away!
[Caption below the panel:] Fun fact: Spinthariscopes have the highest ratio of "that can't possibly be safe and legal" to actual safety and legality of any known toy.

This is another comic with one of Randall's fun facts .
As stated in the comic, a spinthariscope is a device with a small amount of radioactive material ( americium or thorium ) and a screen. When one of the radioactive atoms decays, it emits an alpha particle , which strikes the screen, which emits a small flash of light. You can see these flashes by looking through a lens.
It was invented in 1903 initially as a scientific instrument, but was soon replaced by more accurate and quantitative devices. But the original device was still popular for some time as an educational toy for children, and you can still get them today.
The joke in the comic is that most people have little understanding of radiation, and overreact to any mention that something is radioactive. So when Cueball tells Megan, White Hat, and Ponytail that the toy contains radioactive material, they're shocked and scared. But the amount of radioactive material in the toy is very tiny and the radiation is itself so trivially contained that there's practically no risk from it. The short-ranged alpha particles are likely stopped by the lens through which the harmless flashes of light (from particles that instead hit and neutralise in the internal screen element) are seen. Alpha decay always leads to an unstable decay product, which results in further decay (always gamma decay, and sometimes beta decay as well) which are less easily blocked, but the amount of such radiation from these decay products is negligible.
The fun fact in the caption says that Spinthariscopes have the highest ratio of "that can't possibly be safe and legal" to actual safety and legality of any known toy. When people hear about Spinthariscopes for the first time, they often assume, due to the radioactive material inside, that they must be very dangerous. They thus also question if such a toy is at all legal to make or own in the first place. But the fact is that Spinthariscopes are both safe and legal to make, sell and own.  So, the perceived danger and presumption that it must be illegal is at a very high number, and the actual danger and the actual illegality results in a very low number on the same scale. It is this ratio between perceived and actual danger and illegality that are the highest for Spinthariscopes, higher than for any other known toy.
The formulation, however, causes some confusion, because the caption uses actual safety and legality (high) instead of actual danger and illegality (low). Instead of a high ratio between perceived danger and actual danger, the result is an even ratio between perceived danger and actual safety, which are both high. The ratios for the other mentioned toys would also be even, as they have low perceived danger and low actual safety. This is obviously not the intended meaning, as the other toys are said to be toward the other end of the scale.
The title text mentions some other materials/toys that sound dangerous but aren't. Gallium is a metallic element with a low melting point of 29.76°C (85.568°F) so it will melt in your hand. Additionally, gallium has strange properties when it interacts with aluminum, causing aluminum to "melt" or become brittle. Tritium is a radioactive isotope of hydrogen, but can be used to create glowsticks and other lighted objects. Though these two toys might seem dangerous, they are actually typically used perfectly safely.
At the opposite end of the spectrum is lawn darts , a toy containing large darts that are thrown into the air to fall back down onto a target that's placed or marked upon the ground quite near the players' positions. Unlike the spinthariscope, which sounds dangerous but is actually harmless, lawn darts sound relatively innocent but can cause severe injury if you accidentally hit a person (and a few children were even killed ), so they were banned in the US and Canada in the 1980s. When sharpened, these toys even compare quite favorably to weapons of war .
Today many houses have smoke detectors using ionization caused by radioactive decay of Americium-241 to detect the smoke. So having something with radioactive material in your house is quite common, and in this case increases the safety level for those houses.
[Cueball is holding a small item up in on hand in front of his three friends. Megan has her arms lifted and bent in front of her, White Hat has his arms raised over his head and Ponytail is pointing at Cueball while her other hand, held down behind her, is balled into a fist.] Cueball: It's a spinthariscope, a 1940s toy with a radioactive isotope inside. If you let your eyes adjust to total darkness and look into the lens, you can see the flashes of individual atoms decaying. Megan: What?? White Hat: Aaaaa! Ponytail: Get it away!
[Caption below the panel:] Fun fact: Spinthariscopes have the highest ratio of "that can't possibly be safe and legal" to actual safety and legality of any known toy.
