‘And Isaac spoke to Abraham his father and said, “My father.” And he said, “I'm here, my son.” And he said, “Look, the fire and the wood. But where is the lamb for the burnt offering?” '
The men must previously have wondered the same. It stood out a mile. But Isaac has clearly been pondering over it and now cannot resist the question which must have gone like a dagger into his father's heart. And he is obviously of an age to be aware of the details of such an offering and to be aware that lambs do not just come from nowhere. But it is clear he does not know of the purpose of the visit.
Isaac's question brings out that already as a young lad he is quite familiar with the idea of the sacrificial lamb and that at this stage the ‘burnt offering' (literally ‘that which goes up') is the regular sacrifice.