Nearly every boss has said it. And just about every employee has heard it. Yet it's one of the most meaningless lines ever spoken in the office:

"My door is always open."

The statement usually is followed up with some version of, "If you ever have an issue with anything, please come talk to me."

What's wrong with this? Isn't it important for your employees to know that you are open to hearing their suggestions, concerns, and criticisms? Of course it is.

But let's be real here: In most cases, "My door is always open" isn't really an invitation to speak up. It's a cop-out. It makes the boss feel good but puts the onus entirely on the employees. You might as well say, "You find the problems and then take all the risk of interrupting my day and confronting me about them." How many people have taken you up on that offer?

Your employees have lots of opinions about everything--your strategy and vision; the state of the competition; the quality of your products; the vibe in the workplace. There are tons of things you can learn from them.

But how many of these ideas and opinions have you actually heard? A tiny fraction, I'd bet. The reality is that companies are full of things that are left unspoken. And even when they are out in the open, the CEO is almost always the last to know.

I like to think of myself as a leader whose door is always open. But I recently learned that an open door isn't enough.

As readers of this column know, 37signals recently launched a product, Know Your Company, that is designed to solicit very specific feedback from employees on a regular, non-anonymous basis. The idea is that people don't volunteer information--they release it. And they release it only when they're asked about it. In other words, if you want answers, you have to ask questions. So, for the past few months, I've been asking all of our employees about the way they perceive the company's strategy, decisions, competition, quality, leadership, and the like.

It turns out there was a lot I didn't know. I asked, for example, if people had noticed anything that we had gotten worse at over the past year. The answers were clear: We'd become a less inventive company. With so many people buried in stuff that needs to get done, several people told me, there wasn't enough time to experiment. I have to fix that.

Another question--"Is there anything you worked on recently that you wish you could do over?"--revealed that for some people, successful projects were not being perceived as successes. Why? Because they weren't kept in the loop about how important their work actually was. That's the most serious wake-up call I've received in a while.

Bottom line: Rather than proudly announcing that your door is ajar, get out of your office and knock on your employees' doors instead. And understand that a reluctance to speak up is totally reasonable. Who knows? Maybe you have staff members who were reprimanded at, or even fired from, a previous job for speaking up without being asked.

You need to make it safe to speak up. You'll be surprised by what you hear. You'll be enlightened. In some cases, you may be embarrassed or even ashamed. But you can't know your company unless you know--really know--your employees.