Picture this. You get an email asking for help. Let's say it's all about how to best teach a particular class that you've been teaching for years.
You're happy to help. You love helping. You love teaching this class. You type out a long, multi-paragraph response to each of the many questions posed in the initial email. It's energizing to talk about pedagogy and teaching with a fellow teacher. Woo hoo!
But it turns out that you weren't in a conversation at all. The response to your message? Crickets.

Hello? Is anybody home?
How about another one?
Here's an email from a former student asking about a new research study. You respond warmly with your thoughts on the study and what it means - or doesn't mean - for practitioners. It's wonderful to connect with students outside of school. Yay!
But you start to wonder if the connection's been lost when you never hear back from this person.

Apparently I'm talking to myself.
Now imagine this happening over and over again. Sometimes several times a week*.
Your motivation to respond helpfully and cheerfully to similar requests may start to wane.
I get that many of us are busy.
I've got quite a bit on my plate right now, too. (I imagine that's what's gotten me into the position where I have loads of people asking for help and advice.) And I know that sometimes we mean to respond and just don't. Things slip through the cracks. That's okay. And if this were the occasional, rare occurrence I wouldn't even feel the need to say anything. I totally get it.
But we've gone beyond that level just based on my n=1 inbox stats.
Seriously, people.
This is not okay.
When someone does something nice or helps you out, at a minimum you should let that person know that you've received their message.
It's part of being a human. We help each other. We acknowledge each other.
In a similar example, if someone holds the door open for you when your hands are full, the proper response is to at least mumble a thank you. A quick nod will suffice. A smile. Some small acknowledgement helps perpetuate the behavior and provides a subtle human connection.
Actually, a better example more equivalent ot the email scenarios above is one where you had your hands full, asked someone to help you with the door, and then waltzed right through it without looking, speaking or acknowledging the person in any way.

Boo. Step up your game.
Not okay, right?
If the electronic equivalent of a smile doesn't happen after I respond to your request for help, I assume one of these things is going on:
- I never responded to your message at all. Did I dream it? Did I forgot to hit send? Did my computer/email system develop a horrible, hideous virus that's been blocking emails to only specific people? I have actually gone back to check this in particularly egregious cases, where I'm totally surprised not to have heard anything back.
- You've been involved in a severe accident that prevents computer access. Where should I send your card? Do you need a meal chain? Are you in a ditch on the side of the road somewhere?
- The advice/suggestions that I sent were absolutely the worst ideas you've ever heard. So bad, in fact, that you immediately cut off all contact. Forever.
- You're self-absorbed and/or totally oblivious to the way polite society works. Now that you've gotten what you needed, there's no reason to correspond any further. Let's just hope you don't ever need a recommendation letter.
- You're overwhelmed. Your silence gives the impression that my response is not important to you, even though this isn't true. Somehow life got mixed up to the point where you're too busy to smile when someone figuratively holds the door open for you.
Please don't make me wonder which of these is true in your case.
In summary: Type in six letters. (Hint: t-h-a-n-k-s.) Hit "reply." Find the time. No one is too busy for that.

Or you can express yourself with vegetable pictures. Either way.
* I should mention that many people do respond, and do say "thank you" and do express appreciation and gratitude. Thank you. You all keep a good thing going and make my working life spent mostly behind a computer screen feel a little more connected. You're the best.