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Boiling is when a liquid changes to a gas or vapour through the busting of bubbles.
You will have seen liquids boiling, with the liquid bubbling away. So, if it is bubbling, it is likely to be boiling.
By the way, water, for example, will still evaporate into the air even when it is not boiling, but that is slow. Boiling sends water vapour into the air very quickly.