Before the birth of quantum mechanics scientist Thomas Young performed an experiment called Young’s Experiment which was later renamed the double slit experiment.
The simplified explanation is to visualise a gun firing paintballs through a single slit in front of a chalkboard, if you fired the paintballs you would get a line of paint in a similar pattern to that of the single slit. If you were to add a second slit and perform the same experiment you would create 2 lines on the board that resemble the 2 slits.
Now let’s image you had the same single slit, but instead of paint balls you have the slit immersed in water with the same chalk board behind the slit. If you dropped a pebble into the water behind the slit you would get a wave from the water that would be most intense in the middle point, acting in the same way as the paintballs.
The next step would be to mimic that of the paintballs and create a 2nd slit and drop the pebble in the water. This is where an interference happens and the waves act like waves, with the waves canceling each other out creating an interference pattern. This creates multiple lines on the chalkboard instead of just 2. Different, but nothing to mystical yet.
Now let’s take this to the microscopic level and repeat the paintball experiment with electrons (which are tiny particles of matter). If you repeat the single slit experiment it acts in the exact way you would expect with just a single line. Now this is when it gets weird, if you repeat the double slit with electrons it behaves in the same way as the waves creating multiple lines instead of just 2.
This makes no sense as this is not how matter is supposed to act, but upon repeating the experiment in different ways they repeatedly got the same result. Being baffled by this, the experimenters decided to set up a measuring device in front of one of the slits to watch how the experiment was producing this result. This act of measurement changed the way the particle behaved creating a simple 2 slit pattern as would have been expected in this experiment. This means the simple act of observation on the microscopic level changes the outcome of the experiment, as if the particles know they are being watched.
Watch the video if you would like to get a visual understanding of this experiment: