Does anyone know the answer to this glass illusion?

There are a fair few conjuring tricks on YouTube, but the glass illusion is one of the more frustrating to figure out. Essentially several glasses, ranging from the very small to the very large, are lined up on a work surface. The conjurer then proceeds to pour the liquid from the small glass into the larger glass, each time succeeding in filling it to the top. The trick ends when the largest glass is then used to fill all the previous glasses. Several explanations have been provided by YouTube viewers as to how this is done, the most convincing being that all the larger vessels are in fact two glasses in one and that the liquid is simply poured down the side of the bigger of the two and so never truly ends up filling anything. This sounds like it makes sense, but the demonstration is so inept that it left me even more confused. So if anyone knows the real answer, could they please post it below.