Riddle: What has a heart that never beats, and is green on the…

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im-a-sphere-with-a-shell-in-pairs-i-dwell-once-cracked

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i-start-as-a-seed-but-i-am-not-sown-you-crack-me-open

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i-am-often-cracked-yet-never-spill-i-come-in-dozens-but

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i-have-keys-but-open-no-locks-i-have-space-but-no-room-a60949

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i-can-slow-you-down-or-speed-you-up-but-im-not-time