What is the thickest, most viscous substance known?
Mercury is terrifically dense, but it isn’t viscous. It flows very readily (so much that one of its older names is ‘quicksilver’) and pours like water. Highly viscous fluids are things like tar and molasses, that pour really slowly. Some places will tell you that glass is a highly viscous liquid, which I think is BS but it gets repeated often and it might very well be the answer your teacher wants. Edit: Old glass windows are thicker at the bottom because they were produced in nonuniform thicknesses via a pour or a crown glass process. When they were installed, it was easier to install them with the thick edge down. Some old glass windows, installed by people who weren’t paying attention, are found to be thinner at the bottom. Wikipedia’s article on glass, about a third of the way down, talks about the ‘glass flowing’ idea and is mostly dismissive. Its atoms aren’t arranged in crystals but you’d have to watch glass for much longer than the age of the universe to observe any fluid behav
