In the double-slit experiment, particles such as electrons or photons are typically used. When these particles are directed towards a barrier with two slits, they exhibit wave-like behavior and can pass through both slits simultaneously. This results in an interference pattern on a screen placed behind the slits.
It's important to note that individual particles are not split into two. Instead, they behave as waves and interfere with each other, leading to the observed pattern on the screen. This phenomenon is a fundamental aspect of quantum mechanics and has been extensively studied and confirmed through experiments.