How do I time travel?
See that nice-looking PORTAL in the fountain?
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Just tap it to step through it.
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Each time you step through the portal you will travel back one step. When you've reached the oldest possible time era, stepping through the portal will bring you back to the present era. Time travel as many times as you like.
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The three available eras in the bubble are: the Present, the 50s, and the 1600s. Any time you step through the portal you cycle through those three times.