When you heat up mashed potatoes, the starch molecules start to break down and reorganize, causing them to become thicker and more gel-like. This is why it's essential to add a little bit of liquid when reheating, to help loosen up the starches and make the potatoes creamy again. You can use milk, cream, or even broth to achieve this.
But here's a fun fact: did you know that the type of potato you use can affect how well they reheat? High-starch potatoes like Russet or Idaho are more prone to becoming gluey, while waxy potatoes like Yukon Gold or Red Bliss hold their shape and texture better. It's all about the starch content, baby!