Do not eat any leavened food
One who is careful with the minutest bit of leavened food on Passover is guaranteed not to sin (inadvertently) the entire year. - Rabbi Yitzchok Luria, the `Ari'.
After Rabbi Moshe Horenstein married the sister of Rabbi Sholom DovBer of Lubavitch, he noticed that his new brother-in-law would not use sugar on Passover.
Rabbi Moshe failed to understand why: he himself owned and operated a sugar refinery and knew that no leavened substances are involved in the sugar-making process.
Rabbi Moshe resolved to provide the Rebbe with sugar for Passover.
Despite his confidence that his sugar was 100 percent kosher for Passover, he took extraordinary precautions, purchasing new equipment and taking personal charge of the production every step of the way.
On the day before Passover, he brought the sugar to Rabbi Shalom DovBer. He then proceeded to describe the entire sugar - distillation process, pointing out that there are absolutely no grounds for concern. He added that nevertheless, he, Reb Moshe, had personally overseen the making of these sugar cubes.
As Rabbi Moshe spoke, he noticed the grave look on Rebbe's face.
The more he elaborated, the more serious the Rebbe's expression grew. When Rabbi Moshe finished, the Rebbe took a sugar cube and broke it in two. Out fell a grain of wheat...