Enzymes fit their substrate like a “lock & key”. What does this mean?

An enzyme meant to catalyze bonding would have a “lock” for each substrate “key” at its active site, allowing multiple substrates to bind to it

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At the boiling point, temperature no longer rises with heat added because the energy is once again being used to break intermolecular bonds. Once all water has been boiled to steam, the temperature will continue to rise linearly as heat is added.

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