Heuristics applied when opening a safe lock

When teaching testing I often talk about heuristics. Everyone uses heuristics whether you are aware of them or not. Simply explaining heuristics – A heuristic is a rule of thumb. It’s an approach or a method of how to a solve a problem. It’s important to remember that all heuristics are fallible.

In software development we use heuristics everyday. Your heuristics are build upon your experience. This is why I find it extra interesting comparing my heuristics with those of my children, who have different experiences than I have.

A few years ago when my kids still played with toys, I was in the bathroom getting myself ready for the day when I heard my youngest daughter crying from the other side of the wall. I opened the door and asked her what was going on.

“I can’t open the safe lock and I have all my money in there. We have forgotten the code and now we can’t open it.” Of course I went to see how I could help them.

My oldest daughter was sitting on the floor and furiously shaking the safe. She mumbled something about how she had changed the code a while ago. They had tried several different codes but with out any luck. So I sat down on the floor with my daughters and asked them to show me what happened when they entered the code.

They entered a code. “Error” was shown on the display and the safe made a kind of buzzing sound indicating that something was wrong .

I asked them what different things they had  tried to do to open it. They had actually tried several things like

  • Trying to remove the hinges. Kind of clever I think ( but it might have broken the toy).
  • Searching the internet for how to open the safe.
  • Tried different codes.

Suddenly my youngest shouted, with pride in her voice: “We can use a saw and cut it open!” We chose to proceed without bringing out the saw since they really liked their safe lock.

Opening the safe box


To give you and idea of how the safe is opened, I have provided a short gif and a brief explanation. There are actual sounds made for each action you do but since I can’t upload videos without upgrading my account to Premium this will have to do. You just have to imagine any toy with sounds and I bet you will understand how this quickly can drive you crazy…

  1. First insert a plastic “key” card (sound played).
  2. Press the button “Withdraw” twice. Not sure why it needs to be pressed twice (sound played).
  3. Then enter your code (sound!). Another sound is played when the code is correct. You can then open the safe with the black handle below the “Withdraw” button.

Heuristics applied

Before moving on with some ideas that came into my mind we tried a few codes again but without any luck.

Since the safe box actually had some software I used the “Long press heuristic” hoping I could reset the software. I even tried combining different buttons hoping to trigger some error which would reset the safe. Yes, this has actually happened before!

I then went with the “Google search heuristic”. It had already been tried by my children but I had a deeper knowledge of how to search the internet. This unfortunately didn’t help either and to be frank I was more eager to interact with the safe lock rather than reading.

When this didn’t help, I wanted to use my “Remove the batteries heuristic”. It was a long shot but it was a toy so maybe removing the batteries would reset the code. While doing so I discovered a tiny hole next to where the batteries goes.20210507_184255

You might wonder why I didn’t look for it when considering to reset the safe code – but it just didn’t cross my mind. As a side note there is a cognitive bias called the Hindsight biaswhich is very common and also known as the “I knew it all along” phenomenon. The effect of this bias is that it causes us to overestimate our ability to predict events. And might also be a reason for you thinking – “I would have looked for the reset option the first thing I did.” Maybe you would have – maybe you wouldn’t. 

With excitement in my voice I asked the children to go get me a tooth pick. Again I was using my heuristics – “The press reset with a tooth pick heuristic“. I pushed the tooth pick into the tiny hole. The safe made a new type sound and seemed to be reset. “Wohoo! Could this be it?”

We inserted the plastic card, pressed the withdraw button twice and then pressed the code several times without any luck. The fourth time the safe made a new sound ( described as a sad sound by the children), a light went red and then the display was blank. This was a different behavior! Intriguing!

I noticed that the text on the display was very hard to read. It was not very bright. My youngest used her little hand to shadow the display to make it easier for me to read. I suspected that the batteries might be running low so we changed the batteries. And voilà we could finally open the safe.

My heuristics were build upon my experiences with toys, software, and software and hardware working together. Experiences which my children did not yet have. Some were consciously applied but I am certain there were several unconsciously used heuristics as well. Also by interacting with the toy, exploring different alternatives serendipity might have played a part here too. I still don’t know why the safe wasn’t reset properly after pressing the reset “button”. In hindsight it could have been waiting for a new code to be entered but that hypothesis have yet to be tried.

A toy safe is however different from a real safe. The context of where the heuristic is applied is of great significance.

To read more about heuristics in Software Development and Testing the following article, Software Testing Heuristics: Mind The Gap! by Richard Bradshaw and Sarah Deery is a great starting point containing a long list of references.

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