A textbook mentions: 

> 
> A bulb has a solid filament that is heated. Energy levels in solids overlap, so all energy changes for the electrons are allowed. This means the electrons can emit photons with any energy, producing a continious spectrum when a solid is heated.
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I don't understand what it means by 'overlap of energy levels', and how this could lead to a continious spectrum. Not all solids produce a continuous spectrum, so what does this phrase mean?

Source: ["AQA A Level Physics (Year 1 and Year 2)",
chapter "Line spectra and continuous spectra"](https://books.google.co.uk/books?id=KE2cDwAAQBAJ&pg=SA3-PA5&lpg=SA3-PA5&dq)