Idol worship is rooted in a profound absurdity and a basic lack of logic. A person who creates an idol fails to engage his intellect, which is the core of human understanding [אבן עזרא]. Recognizing the foolishness of this behavior does not require deep knowledge or complex intellectual proofs. A person needs only simple, honest reflection to see the clear error of his ways [מלבי״ם].
The irrationality becomes obvious when looking at how the materials are actually used. A person takes a piece of wood and uses half of it for his everyday physical needs. He burns it in a fire to stay warm, bakes bread directly on the coals [רד״ק]—with the dough being called bread in anticipation of its final baked state [מצודת דוד]—and roasts meat. Afterward, he takes whatever piece of wood happens to be left over [אבן עזרא, רד״ק, מצודת ציון]. A sharp irony is at play here. Instead of dedicating the main part of the wood to his god from the very beginning, he serves his own physical appetites first and uses only the random leftover scraps for his worship [מלבי״ם, ביאור שטיינזלץ].
He then transforms this scrap into an object of worship. The primary approach among commentators is that this resulting object is a detestable idol, strictly forbidden by God [רד״ק, מצודת דוד, מלבי״ם]. A different approach highlights the irony of the situation, viewing the object as something absurdly set apart. After freely enjoying the wood for mundane daily tasks, the person takes the useless excess and ridiculously proclaims it a holy item that is now forbidden for regular use [שד״ל, ביאור שטיינזלץ]. Finally, he bows down to this block of wood. The exact nature of this block is debated. The primary approach among commentators is that it refers simply to a natural branch or piece of produce [אבן עזרא, רד״ק, מצודת ציון, שד״ל], though some explain it as a specifically carved shape [אבן עזרא]. Another perspective emphasizes the sheer worthlessness of the material, describing the object of worship as nothing more than decayed, rotting wood [רש״י, מלבי״ם].