Something interesting/absolutely uselessly nerdy: There exists a radiopharmaceutical called fluorodeoxyglucose (FDG, naturally). It's a radiotracer (meaning we can follow it as it goes through the body) and is a glucose analog which follows the same uptake pathway as glucose does in the human body - with one small caveat. FDG passes through the first enzyme of the metabolic pathway (hexokinase) fine, but, where glucose would enter the glycolytic pathway to be used as energy, it gets rejected. This leaves it floating in a sort of limbo (just chillin in the intracellular space) and allows PET/CT scanners to create a 3D map of where it accumulates. In a sense, it tricks tissues into trying to metabolize it which, incidentally, makes it super useful in identifying tumors (because they love using up tons of the body's resources)! Almost everything in the body gets tricked by FDG, this even includes the brain...But the kidneys, somehow, know that FDG is garbage. They pull out as much FDG from the body as they can and pass it as waste. (seen in the following image from Nature Reviews: FDG in tumors at the top, designated by the purple and green arrows, urine at the bottom designated by the yellow arrow) [RIP EMBEDDED IMAGE]
How is it that they can tell the difference?