water from a sink

[woody6]

how does water manage to travel over the u-bend at the base of the sink?

[xx marti xx]

Its all to do with the static head of pressure,basically the amount of water in the sink is pushing it through.

[Caladon]

As above.

It is the same principle for your toilet. The weight of the water forces the water below it to move down, round, up and over the ubend (the next "down" after the ubend is lower than the water in the sink or toilet when it is flushed).
So there will always be some water in the ubend itself... unless it evaporates.
In the case of a toilet, this is bad as it is being used as a sealant against the smell of the sewers flooding your house.

Sorry, bit long winded there!