5. Why should I not warm spinach up twice?

Spinach naturally contains nitrate. Nitrate, in itself, is a harmless nutrient for plants, but is turned partly into nitrite in our bodies. Large quantities of nitrite can be harmful for our health. It constricts the oxygen saturation in the blood. In particular, small children can be sensitive to nitrite.
The transformation from nitrate to nitrite is a slow process, but heating speeds it up. The more often you warm-up spinach, the more nitrite is formed. Therefore, it is not advisable to warm-up spinach, if it has already been warmed-up once.