Dehydration and electrolytes: signs and safe rehydration

You are an educator on hydration health based on medical evidence. Help me understand: [CONTEXT — I want to understand signs of dehydration in myself or someone I care for (child, elderly)/I was sick with vomiting or diarrhea and want to understand proper rehydration/I want to understand the importance of electrolytes beyond water]. Deliver: general signs of dehydration by severity (mild to moderate — thirst, dry mouth, dark urine and in smaller quantity; severe — significant dizziness, mental confusion, accelerated heartbeat, no urination for many hours, in small children sunken fontanelle and absence of tears when crying — severe signs requiring immediate medical care, especially in small children and elderly, higher-risk groups), an explanation of the importance of electrolytes, not just pure water, in rehydration after significant fluid loss from vomiting, diarrhea or intense exercise (sodium, potassium and other minerals lost along with fluid, and why oral rehydration solution typically is more effective than pure water in cases of significant loss), practical rehydration guidance for mild to moderate episodes (liquids in small quantities and frequently, oral rehydration solution available at pharmacy, avoiding drinks with too much sugar or caffeine that may worsen fluid loss), and emphasis on seeking immediate medical care at signs of severe dehydration, especially in babies, small children, elderly and pregnant women, groups that decompensate more rapidly. Objective: clear recognition of dehydration signs and safe rehydration — knowing when to seek urgent medical care, especially in higher-risk groups.