If you’ve made hot cocoa with powder, you’ve probably experienced the dark sludge at the bottom of the cup. This chocolate goop seems unavoidable, despite being absolutely certain that you dissolved all the powder when you first stirred your drink. For years I thought I was just a bad stirrer, but it turns out that the sludge actually forms as the drink cools down. As the hot cocoa cools, the solubility of the hot chocolate powder is reduced. This means that the amount of powder you can dissolve in a mug full of water or milk is lessened. So, as the temperature falls, the powder precipitates and collects at the bottom of your mug leaving the hard-to-clean but delicious chocolate goop.