Why is a tie always possible in Senate votes?

Answer 1

The Vice President of the US votes to break the tie. In Article 1 Section 3 of the Constitution it says that the Vice President becomes the President of the Senate, but has no vote, unless there it a tie.

Answer 2

A tie is possible in the Senate only when an even number of Senators vote. The Senate seats two members for each state, or 100 total; however, members are sometimes absent or abstain from voting, so it's incorrect to say a tie is always possible.