Portable receptacles, such as bathtubs, whirlpools or Jacuzzis, can therefore never function as mikvehs. The gym offers whirlpools, saunas, and massage therapists. Many small whirlpools and eddies can form in its waters. This swift current can lead to standing waves, large whirlpools, and roiling eddies. "Carnival Glory" has two pools, six whirlpools, and a water slide. The before-mentioned potholes were created starting with sand and gravel caught in eddies or whirlpools. These facts are true due to the beating cilia creating whirlpools, or vortices. Small whirlpools form when a bath or a sink is draining. The rapid and irregular currents, whirlpools, and winds of the Messina Strait hampered navigation. These features combine to create whirlpools, standing waves and a variety of other surface effects. Navigation on the river is difficult because of a number of rifts, rapids and whirlpools. In both modes there are a variety of obstacles including ramps, shallow water and whirlpools. Large cataracts, such as Niagara Falls, produce strong whirlpools. Examples of this fact are the shapes of tornadoes and drain whirlpools. There are a few minigames, which involve guiding the boat away from rocks and whirlpools.