Why can hydrogen gas produced by alkali metals and water ignite?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

The reaction between alkali metals and water is highly exothermic, meaning it releases a significant amount of heat. This reaction produces hydrogen gas as a byproduct.1

The hydrogen gas can ignite because the heat generated by the initial reaction is sufficient to raise the temperature of the hydrogen and the surrounding environment. When this hydrogen mixes with oxygen in the air, it can ignite or explode, further increasing the heat produced by the process. The intensity of this reaction often depends on the surface area of the metal exposed to the water; for instance, small droplets or thin films of alkali metal can lead to explosive results.12