In the video below, you see three rings of coloured dots. In each ring there is one gap (a missing dot), and these gaps rotate like the arms on a clock. So far nothing remarkable. But see what happens if you fixate on the central cross for 15 seconds. Illusory dots of various colours will start to appear where the gaps are!

This illusion is a demonstration of the colour after effect. After effects are very basic phenomena, and most of the video is essentially decoration, not necessary for the illusion occur. In fact, you will even get a colour after effect if you present a single coloured dot, look at it for a while, and then remove it. If the dot is green, like the inner circle in the video, you will observe an after effect in the form of an illusory pinkish dot.

So what's going on here?

Let's start with the fundamentals. Light is electromagnetic radiation, just like radio signals, WiFi, microwave radiation, etc. Different forms of electromagnetic radiation are characterized by different wavelengths, and visible light corresponds to a tiny range from roughly 390 to 750 nanometre (one billionth of a meter). Within the spectrum of visible light, different wavelengths correspond to different colours: Short wavelengths are blue(ish), long wavelengths are red(dish).

Like most people, I learned about the relationship between colour and wavelength during physics class in high school (see [1]). And I distinctly remember that I found this very puzzling. After all …

I enjoy a good puzzle every now and again. Particularly one that seems very easy... Until you try to solve it.

Below are a four classic questions/ puzzles/ problems. If you've read a bit about problem solving before, you might know some of them. But I hope that you will nevertheless find it interesting to revisit them. (And perhaps make the same mistakes as before!) And if you don't know them, you're in for a treat!

The ball and the bat

Let's start with an easy one. As you can tell by the prices, this question is from way back:

A bat and a baseball together cost $1.10. The bat costs one dollar more than the ball. How much does the ball cost?

This is just a warm-up exercise, and you will probably have answered correctly. (If not, shame on you!) But perhaps you noticed that you almost gave the wrong answer. Your intuition (almost) seduced you with an easy, but incorrect answer. Quite possibly, your train of thought was something like this:

Right, obviously the answer is $0.10. But wait... that's too easy. There must be more to it. Oh, I see now. How stupid of me, the correct answer is of course (...).

So the context in which the question was asked (a post about tricky puzzles) made you think twice before answering. And this dramatically increased your chance of answering correctly. But the amazing thing is that, when embedded in a stream of questions, almost everybody …

'Open access' is somewhat of a buzzword within academia. But unfortunately the term is widely misunderstood. All too often, the label 'open access' is used to refer to any content that can be accessed online for free. By this (incorrect) definition, most websites would be open access, because they offer content that you can read (or watch, listen to, etc.) for free. But you are usually not allowed to reproduce that content, or to sell it, modify it, etc. The only thing that you, as a consumer, are allowed to do is view the website right then and there, for as long as the owner of the website permits you.

This may be free of charge. It may even be a nice gesture. But it is not open access.

The definition of 'open access' is still evolving, but by consensus a number of conditions need to be satisfied for it to apply. In PLoS Biology, Michael Carroll gives us the following list:

Full open access content is

• Easily accessible online
• Available to anyone free of charge
• Available for re-use without restriction except that attribution be given to the source.

No one of these alone qualifies content for an open access label.

The paper by Carroll satisfies these criteria. I downloaded it for free. I am allowed to reproduce and redistribute the paper in it's entirety (here it is). And so are you. I didn't need Carroll's permission to upload his paper. And I do not (have to) care if he …

OpenSesame 0.26 Earnest Einstein, the next version of the graphical experiment builder, will be released soon! For those eager to test drive the next version, pre-release packages are available. You are encouraged to report bugs on the forum.

• Download pre-release packages (choose latest)

For this release, the focus is on bug-fixes, improved stability, and adding polish to the user interface. I expect 0.26 to be the best, most stable release of OpenSesame yet! Also, I'm glad to say that OpenSesame is rapidly becoming a collaborative effort. Special thanks go out to Daniel Schreij, for help in many areas, and Edwin Dalmaijer, for building the Python portable packages.

In the animation below, you can see a ring of four faces gradually morphing between Leighton Meester and Natalie Portman. If you keep your eyes fixated on the green cross in the center, you will notice that the morphing is quite difficult to see—but only when the faces are moving. If you match the movement with your eyes, or if the faces stop moving, you can clearly see the morphing happening.

This illusion was presented twice, in different guises, in the finale of last year's vision sciences illusion contest. Once by Jordan Suchow and George Alvarez, who ended up winning the contest. And once by Rob van Lier and Arnon Koning. So this illusion may not be entirely original, but it is certainly the most aesthetically pleasing rendition out there!

So what's going on here? Previously, I explained this phenomenon in terms of retinal motion. When the faces move, they slide across your retina (the light sensitive part of your eye), at least if you keep your eyes still. So the same face will be 'seen' successively by different parts of your retina. And because the organization of the retina is roughly preserved in visual areas of the brain, the same face will be successively processed by slightly different (sub)areas of your brain. According to the 'retinal motion' explanation, these different brain areas do not communicate effectively enough for changes to be detected. At least not if they are small and gradual.

But, as it turns out, this …