das gekochte Ei, das kochende Wasser: participles used as adjectives

Both participles can stand in front of a noun, and the only thing separating them is timing. The past one says the action HAS HAPPENED to the thing: das gekochte Ei, the boiled egg. The present one, which is the infinitive plus a d, says the thing IS DOING it: das kochende Wasser, the boiling water. Both decline like any adjective.

The usual slip. Mixing the two up, because in your language they look almost the same and in German they differ by a syllable. And the second mistake, which is even more common: forgetting the ending. Even though it comes from a verb, in front of a noun it is an adjective and declines, das gekochte Ei but ein gekochtes Ei.

Why it catches you out. English does both easily —«the boiled egg», «the boiling water»— and even puts them in the same slot. So the meaning is free for you. All that is new is the ending, because English participles used as adjectives never change: boiled is boiled wherever it stands.