Pectins are long-chain sugars that are found in the cell walls of plants, most especially in the walls of fruit cells. There they create a sort of elastic, moisture-retaining barrier and also function as a glue that holds the cells together. Pectins are especially abundant just before fruits are at peak ripeness. When fruit is cut up and/or mashed and then immersed in hot water the pectins come loose, dissolve and disperse. Under the right conditions those sugars can be brought back together into a flow-preventing network, but it takes a little coaxing since pectins repel each other in pure water. Acid generally does the trick as it changes the molecules’ polarity and encourages them to bond.