Leviticus 11:36
Nevertheless a fountain or pit, wherein there is plenty of water, shall be clean: but that which toucheth their carcase shall be unclean. Nevertheless a fountain or pit, wherein there is plenty of water, shall be clean: but that which toucheth their carcase shall be unclean.
The verse belongs to the laws that distinguish between clean and unclean and regulate contact with carcasses, given as health and sanitation laws to teach that distinction and to protect and preserve health through obedience. It makes an exception for a fountain or pit containing plenty of water, which remains clean, while that which touches a carcass becomes unclean. The surrounding chapter pronounces certain meats unclean by the law of Moses and forbids eating any unclean thing, including swine's flesh and the flesh of certain other animals.
Permitted meat was required to come from animals in good condition, with the blood drained. Eating unclean things fills the blood with impurities and shortens life. The food appointed to Daniel and his companions at the king's table included such unclean meats, and eating them would have defiled Daniel. So they refused the king's meats and wines because they had been taught that this class of food would not keep mind and body in the best condition of health to serve God, to honor and glorify Him, to perfect character, and to stand with clear intellect as representatives of true religion.
The requirements did not command straining all water or drinking water to remove the smallest insect, speck, or smallest thing that might be classed with unclean animals. The Pharisees extended the cleanliness rules beyond what was specified into unwarranted extremes and trivial external demands, requiring the straining of all water used and burdening the people with continual anxiety over ritual purity. This perversion was exposed by contrasting the trivial exactions over such small uncleanness with the magnitude of actual sins.