Once a cake, loaf, or batch of cookies comes out of the oven dry, that moisture is gone for good — you can't rehydrate baked starch without turning the crumb gummy or soggy in the process. What actually helps is figuring out which step drove the moisture out, because the fix is almost never longer cooling time or a splash of milk stirred into next time's batter.
An oven doesn't know when to stop drying something out
A cake batter is close to 35–40% water. The moment it hits oven heat, that water starts moving two ways at once: some of it gelatinizes the starch and coagulates the egg and gluten proteins into a fixed structure, and the rest just evaporates off the surface as steam. The structure-setting part happens fast, somewhere around 160–180°F (71–82°C) internal. Once it's set, the crumb's shape is locked in — permanently, the way a cooked egg white doesn't go back to being runny.
Evaporation doesn't stop when the structure does. It keeps going for as long as the item sits in a hot oven, because heat and dry air pull water out of anything porous regardless of whether that water is still doing useful work. So the first few minutes of baking build the cake. Every minute after the structure has already set is just an oven acting like a dehydrator, with nothing left for the lost water to do.
Overbaked and underbaked look almost identical from outside
A toothpick coming out with a few moist crumbs stuck to it means the structure has set and the item is done. A toothpick coming out completely clean, with no crumbs at all, usually means it's already past that point — the last of the surface moisture has evaporated off the crumb along with everything the structure didn't need. That's a narrow window, often two or three minutes wide on a home oven.
Cookies are trickier because the visual cue lags the internal one. The center looks underdone at the exact moment it needs to come out, because carryover heat — residual warmth still moving through the dough and the hot baking sheet under it — keeps cooking the center for another minute or two after the oven door opens.
Your oven is probably not the temperature it says it is
Most home ovens run 25 to 50°F (14 to 28°C) off their dial, and they drift further as the thermostat ages. An oven running hot bakes faster than the recipe's stated time assumes, which means a timer set for the recipe's number pulls the item out too late relative to when its structure actually finished setting. An oven thermometer sitting on the middle rack is the only way to know which direction yours is off, and it costs less than the ingredients in most of what goes wrong because of it.
Can a dry batch be saved
Not by putting the water back — there's no way to reverse what already evaporated without oversaturating the crumb from the outside in, which just trades dry for gummy. What works instead is adding moisture somewhere the structure can absorb it without collapsing.
For cake, brush on a simple syrup — equal parts sugar and water, heated just until the sugar dissolves — while the cake is still warm. It soaks in along the same pores the water evaporated from and brings back both moisture and sweetness. For dry cookies, dunking in milk or coffee does the same job by the glass instead of the brush. For bread that's gone past saving as a loaf, turn it into something that's supposed to be soaked or toasted anyway — French toast, bread pudding, panzanella, or croutons all use dryness as an ingredient rather than fighting it.
None of that undoes the bake. It just gives the dry result somewhere useful to go.