It's likely a Presta valve (vs. the Schraeder valve like you find on cars). An adapter like these is a cheap way to use a typical compressor like at gas stations, otherwise most bicycle floor pumps sold these days are compatible with either type of valve. Another thing is that Presta valves, in addition to any plastic cap, have a little tip at the end that needs to be unscrewed to put air in (or out), then you screw it back in when you're done.

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