The cell membranes of the plants and other organisms are semi - permeable. This means that they allow other substances to get in and out while ban others. In this item, placing lettuce or spinach in water, allow water to get into the cell through the process called pinocytosis or "cell drinking". This process makes the leaves crispy.
Placing lettuce or spinach in water allows water to get into the cell through the process called osmosis, which is basically the facilitated diffusion of water. The water diffuses into the spinach or lettuce, making them crispy.
