This subject is important and has been brought up before. I know when I hike, I say hi to nearly everyone on the trail. I will even ask if they are okay. You can get a sense of how a person is feeling by watching for a few moments. If I see anything, like a person walking slowly, head down, possibly a bit disoriented, I will just ask them if they need anything, if they have enough water, are they hiking with others. It's all just casual questions. Their answers are when I know. I've only helped one hiker on the trail and I knew there was a problem on the second question, "do you have water"? They said yes and showed me a nearly empty bottle. I told them I would stay with them until I could filter some water for them. I stayed with the person even after I filtered water for them.
Do I get answers from people who may be irritated with me? Sure, just ask Gary when I asked a couple of girls on the trail if they had enough water going up. They looked at me like I had asked something personal and strongly said..."we're just going to Lone Pine Lake". I really don't care if people think I'm prying. I'd rather piss 'em off a bit then find out later that I should have helped.

"Turtles, Frogs & other Environmental Sculpture"
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