Here, there are two things coming into play; air and surface tension.
Firstly, it is air that usually makes liquid fall out of beakers (with help from gravity of course!). Air rushes in, and this pushes the liquid out. Here we are stopping air from getting into the beaker by making a 'water barrier', and this 'water barrier' is all down to the surface tension. Surface tension doesn't mean that water is tense and stressed like a Lab Rat that can't find his shoes. No! Instead it means that the particles that make up water are sticky and they like to hang out together and so they can make a sort of skin, or barrier.
This means that when you turn the beaker and postcard upside down, the water clings onto the postcard and fills in the tiny gaps between it and the beaker. This 'water barrier' blocks air from getting in the beaker, and with no air getting in, no water can be pushed out, so the water and the postcard stay put.
This one is best done outside as you don't want any wet patches on the carpet!
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