Gravity-fed toilets are the most common type of toilet. As the water height rises in the bowl, gravity pulls it back down, which pushes the water into the up-leg of the trapway.
Also See:
Dual Flush Toilets
Pressure Assisted Toilets
Jetted Siphon Action Toilets
High Performance Toilets
How Toilets Work
History of Flushing
Trapway
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