In a glass of Champagne, why do bubble trains often rise from the same tiny spots instead of appearing evenly everywhere?

Answer: Dust and scratches seed them

Dust and scratches seed them ✓ — Right: bubbles need starting places. Tiny fibers, dust, or glass imperfections can hold microscopic gas pockets, and those pockets act like reusable launch pads for bubble trains. That is why some sparkling-wine glasses are deliberately laser-etched at the bottom: the flaw is engineered so the bubbling looks steady.

Warm spots release gas — Warm patches sound plausible because gases often escape more readily when conditions change. But temperature differences would not make neat, repeated launch pads fixed to the same points on a glass. Those steady streams come from tiny trapped gas pockets on fibers, dust, or surface flaws.

Settled sugar crystals — Sugar crystals would be a plausible guess because crystals can seed bubbles in some kitchen situations, but Champagne sugar is dissolved, not sitting as grains on the glass. Many Brut Champagnes contain relatively little residual sugar and still show strong bubble trains. The real pattern comes from tiny gas pockets on fibers or rough spots.