In a psychological experiment first designed by Wolfgang Köhler, people are asked to choose which of these shapes is named Booba and which is named Kiki. 95% to 98% of people choose Kiki for the angular shape and Booba for the rounded shape.
It is thought that this has implications for language development, in that the naming of objects is not completely arbitrary. The rounded shape may most commonly be named Booba because the mouth makes a more rounded shape to produce that sound. Similarly a more taut, angular mouth shape is needed to make the sound Kiki. The sounds of a K are harder and more forceful than those of a B, as well.
Note also that, in the Roman alphabet, the angular shape mimics the angular letters K and I, while the rounded shape mimics the rounded letters B and O.
The kiki visual shape has a sharp inflection and the sound ‘kiki’ represented in your auditory cortex, in the hearing centers of your brain, also has a sharp sudden inflection. Your brain performs a cross-modal synesthetic abstraction, recognizing that common property of jaggedness, extracting it, and so reaching the conclusion that they are both kiki.