Related על פסחים 2:8

One may not knead matzah using boiling water because it parboils (i.e., causes the outer shell of the wheat kernel to leaven, see Y. Pes. II.7.4, Guggenheimer), nor using lukewarm water because it causes it to ferment, but one may knead it using cold water. One may not knead matzah using any other type of liquid, and if [one does begin to so] knead it, he may bake it immediately [without rendering it chametz], but Rabbi Meir forbids it. But one may spread any of these [liquids] on it (i.e., on the surface of the matzah dough, see Pes. 36a:9).

Water that was used by bakers may be poured out in a level place, but it may not be poured out in a place with a downward slope (קטפרוס) because it gathers together and becomes leavened.