The mechanism of whirlpools

What causes tides?

Tidal currents cause the whirlpools at the Naruto Straits. The tide flows in and out due to the gravitational effect of the moon and the sun on the Ocean. As the earth rotates around its axis each day, the high tide alternates with the ebb tide twice a day, making each cycle about 6 hours long.

What causes tides?