Why do valves stick only when engine starts to warm up and not cold?

Valves may stick when an engine starts to warm up due to thermal expansion, which can cause components to fit more tightly than when cold. As the engine heats, oil viscosity decreases, potentially leading to reduced lubrication and increased friction, especially if there is oil sludge or contamination. Additionally, heat can cause changes in the characteristics of valve guides or seals, leading to binding or sticking. Cold engines generally have tighter tolerances, but the lubricating properties of oil are more effective at lower temperatures, reducing the likelihood of sticking.