system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes things transparent (glass, water, nylon, etc.)?
gpt
Light comes in packets of energy of certain sizes, and when a packet hits an atom if it is of the same 'size' as that particular atom the light packet gets absorbed. If it is not the same 'size' (the atom is too 'big', for example) then the packet is allowed to carry on unhindered and pass out the other side.

Non-EL15: Energy is quantized, meaning it only comes in discrete quantities. When a photon strikes an atom if it has exactly enough energy to make an electron 'jump' up to the next energy level then the photon will be absorbed to do exactly this. In transparent materials like glass, the 'band gap' of the electrons in the material is simply too big for the photon to provide the energy to make an electron jump, so the light carries on straight through.