Any two LEGO bricks in the world will fit together precisely, due to their manufacturing quality and the way they were designed.
Using this principle, store-bought LEGO bricks were modified to transport fluid in precise ways, snapping together to form different circuits for microfluidics, or fluids handled at small, micro-scales. At small dimensions, below a mm, viscous forces overwhelm inertia, and fluid doesn't swirl and slosh the same way as when in a large container, like a bucket. Using this enhanced control at small scales, microfluidics is used for biological applications, blood tests, diagnostics, water quality testing, and chemical and biology research, among other things.
Though it seems simple, the way that these blocks fit together is very important to ensure the entire system functions appropriately, making the injection-molded LEGO bricks more suitable than even 3D-printed pieces.