Resistance random access memory (RRAM) is a rapidly developing emergent nanotechnology. For practical applications and basic understanding, it is important to ascertain whether RRAM undergoes uniform or filamentary switching, but on this point previous area-scaling studies have often shown ambiguous and conflicting findings. Here we demonstrate a simple test—physically breaking the device into two and studying their characteristics individually—can make a definitive determination. Our experiment on two prototypical RRAMs found that one (the nanometallic memory) switches and conducts uniformly while the other (the filamentary memory) does not. It also probes the statistics of nanofilaments: the resistance statistics of the filamentary memory reveals for the first time a large population of partially developed filaments in addition to the filament that dominates switching. Remarkably, the filamentary memory can also be stress-switched to a lower resistance state during fracture, which is reminiscent of stress-switching of the nanometallic memory and may be taken as direct evidence of electron–phonon interaction in the filaments.