Why do we say that a photon is a particle if it is massless? If light is made of photon particles and the photon doesn't have any mass but it is a form of energy (according to my thinking) then why do we call photons particles?
 A: One should probably not say that a photon is a particle (or that it is something at all), but rather that it can successfully be described using particle properties in some circumstances. 
A particle can have several properties (e.g. mass), but for a photon, this does not apply. Particle properties of a photon might include indivisibility and a position in space - but only after a measurement.
One property that all particles share is that they are (in principle) countable, this is done in single photon experiments.
A: In practical terms, we call photons "particles" because careful experiments reveal that they are 1) emitted singly from sources like excited atoms, 2) interact singly with things like photoelectric detectors, and 3) possess repeatably well-defined energies. All these things mean that photons can be considered particles in some circumstances (usually where they can be studied one at a time), despite the fact that they also possess wavelike properties in other circumstances (usually where there are many of them present). 
A: Light when interacts with matter behaves as it made of particles just like when alpha particles collides with an electron and absorbs the energy of the electron,  there is a similar kind of phenomenon when light interacts with matter as in photoelectric effect when an electron of the cathode absorbs energy from light and comes out of the metal.   This phenomenon is same as any kind of particle particle interaction,  so  it is assumed that light  consists of particles when it interacts with matter and this particle is known as photon.  There are no rules to define a particle on the basis of mass
