Question

How can glass be a liquid if it's so hard?​

Answer 1

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The reason for this is glass is amorphous solid.Generally this amorphous solid don't have tendency to flow by itself. So glass is called supercooled liquid. So when you super colled it the arrangement will be like crystalline substance. So the glass become hard enough.

So when thermodynamics variable such as volume, entropy and enthalpy are losses their range then the glass transition takes place then we consider it as liquid.

Answer 2

Glass is hard, it must be a supercooled liquid. Glass, however, is actually neither a liquid—supercooled or otherwise—nor a solid. ... When glass is made, the material (often containing silica) is quickly cooled from its liquid state but does not solidify when its temperature drops below its melting point.