When Avraham takes Yitzchak to the Akeida, the pasuk reads וַיִּקַּ֣ח בְּיָד֔וֹ אֶת־הָאֵ֖שׁ וְאֶת־הַֽמַּאֲכֶ֑לֶת even though, earlier the only thing that Avraham seems to have taken with him was wood (pasuk 3). The text does not read "vayikah eish uma'achelet (or v...I don't know)". The definite article "ha" (not to mention the apparently superfluous "et") is presented introducing each noun (knife and fire) seeming to point to a specific (known?) item though none was already specified. Is there any explanation for what specific knife and fire he took?
According to Be'er Mayim Chayim on this verse, the "hei" in connection with the fire signifies the fire מן ההדיוט that we know from elsewhere (Eruvin 63a citing Vayikra 1:7) has to be brought.
(He doesn't explain the "hei" in connection with the knife. Perhaps, in the same vein, it means "the knife" of the kind we know from elsewhere that has to be used for shechitah.)