Kidney stones are caused by a build-up of crystal-forming substances in the urine. The substances can be calcium, oxolate and uric acid.Sometimes your urine can lack the things it need to stop them sticking together,causing kidney stones. These liquids can cause a lot of discomfort in the kidney area. The ways that they can come out is if you poop them out,you drink lots of fluids like water and if they don't pass then you might need to schedule a removal procedure.