Why does the scale go up after a hard workout?

Almost always water, and it has nothing to do with fat.

Several mechanisms push weight up temporarily after intense training:

Glycogen and its water. Muscles store carbohydrate as glycogen, and each gram of glycogen is stored with roughly three grams of water. Refuelling after a depleting session can add a kilogram or more of weight that is almost entirely water bound to stored fuel.

Inflammation and repair. Training causes microscopic muscle damage, and the repair response draws fluid into the tissue. This is a normal, temporary part of adaptation, and it is most pronounced after an unfamiliar session — which is exactly when people weigh themselves hoping for good news.

Sodium and rehydration. Drinking heavily and taking on electrolytes after a sweaty session shifts fluid balance for a day or two.

Delayed rehydration reads the other way. If you weighed yourself immediately after training, you were dehydrated and artificially light. The "gain" the next morning is partly you returning to normal.

Why this matters practically: daily weight fluctuates by one to two kilograms from fluid, food volume, sodium, glycogen and hormonal cycles. A single reading tells you almost nothing. Fat gain of a kilogram requires an energy surplus of roughly 7,700 calories, which a workout cannot produce.

Weigh consistently, and read the trend. Same time of day, ideally first thing in the morning after using the bathroom, under the same conditions. Then look at a rolling weekly average rather than any individual number. Over several weeks the water noise cancels out and the underlying direction becomes visible.

Measurements, photographs and how clothes fit are often more informative than the scale alone.