If a girl lays her head on a guy's arm, what does it mean?

One of my friends was riding shotgun in the car while I was driving. My right arm was laying on the arm rest and she laid her head on my arm twice. The first time I made a funny comment, and she laughed and kept her head their for a while. The second time was because we were parked and another car came driving toward us. She did it again, saying she didn't want them to see her. She's been really flirty with me lately.

Is this a sign she likes me?

If a girl lays her head on a guy's arm, what does it mean?
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