TSK
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"'Seven days you shall eat unleavened bread; even the first day you shall put away yeast out of your houses, for whoever eats leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.
Then they understood that he didn't tell them to beware of the yeast of bread, but of the teaching of the Pharisees and Sadducees.
Meanwhile, when a multitude of many thousands had gathered together, so much so that they trampled on each other, he began to tell his disciples first of all, "Beware of the yeast of the Pharisees, which is hypocrisy.
The disciples therefore said one to another, "Has anyone brought him something to eat?"
A little yeast grows through the whole lump.