>>11164263particle is in a state. Depending on how you look at that state, it is a linear combination of some number of basis states. Basis states are chosen so that they are _eigenfunctions_ of certain operators: that when applied to the state only affect the overall "size" of the state, not its shape. The thing is, what may consist of only a single basis state for one operator may be a weighted sum, or superposition, of several basis states with respect to a different operator. But when an operator applies to a superposition it picks out one and only one of these states and collapses the wavefunction to that state, because while each basis state is an eigenfunction of the operator, the superposition is in general not. But subsequent applications of different operators may further alter the wavefunction if the eigenfunction of the previously applied operator that the wavefunction collapses to isn't an operator of the second.
The problem is that these applications of operators only correspond to measurements in theory, in practice measurement is more complicated/nuanced, and the question of "what is measurement" is a big one.
