Bakers, food service workers, and sandwich enthusiasts know the heartbreak of stale bread. A top scientist investigated this all the way back in the 1800s. We'll tell you his conclusions.
Jean Baptiste Boussingault is one of the reasons you are well-fed today. A 19th century agricultural scientist, he came up with modern ideas of crop rotation, how important nitrogen was to soil, how best to feed cattle, and how photosynthesis can be measured. He revolutionized farming. Then, because he was French, he took a trip into the kitchen to figure out how to keep all that newly-available food tasty.
Bread gets stale. This has been a source of annoyance to people from time immemorial, and Boussingault aimed to figure out why it happened. Stale bread has a dry feel to it, so most people assumed that loss of moisture, especially water, led to the texture of stale bread. This is not strictly true, and Boussingault proved it. He hermetically sealed a loaf of bread, making sure it lost no water through evaporation - or any other process. It still got stale. Keeping the bread in a moist environment, or moistening it with water on the crust just prior to eating it, won't help the texture.
What will help is heating the bread up, and this explains why bread goes stale in the first place. In bread dough, the starches have a crystalline structure. Heat the dough, and the starches absorb water molecules and change into a kind of gel. Once the heat dissipates, the starches hold the water for a while, then go through a process called retrogradation. During retrogradation the starches give up the water molecules and realign as crystal structures.
Although this can happen more quickly when bread is left out where water molecules are easily stripped away, the presence of water won't prevent it. Keeping the water in the bread (as Boussingault did) won't stop the starches from undergoing retrogradation and realigning. Moistening the bread, once it's stale, won't make the starch take on water. The only thing that helps, temporarily, is heating the bread so the molecules will re-gel. As soon as the bread re-cools the starches will re-crystalize, so eat it while it's hot.