Step 1: Understanding the Concept: 
Dehydration is a condition caused by the loss of too much fluid from the body. It happens when you lose more fluids than you take in, and your body doesn't have enough water and other fluids to carry out its normal functions. Crucially, this loss includes not just water but also essential salts called electrolytes. 
Step 2: Detailed Explanation: 
- While any form of water (boiled, cold) or a liquid diet will help rehydrate to some extent, they primarily replace the lost water. 
- ORS (Oral Rehydration Solution) is the most effective treatment. It is a special mixture of clean water, sugar, and salt. It is specifically formulated to replace both the water and the electrolytes (like sodium and potassium) that the body has lost, in the correct proportions for optimal absorption. This makes it the standard medical recommendation for treating dehydration, especially when caused by diarrhea or vomiting. 
Step 3: Final Answer: 
In case of dehydration, ORS should be given as it effectively replenishes both lost water and electrolytes.