Neccessary & Proper Clause
Article 1, Section 8, Clause 18
Defintion of Neccessary and Proper Clause:
Under Article I, Section 8 of the Constitution, Congress has the power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or any Department or Officer thereof"

What is it like?
The Necessary and Proper Clause is congress having the power to make laws and then execute them. It is also known as the "Elastic Clause" because it has allowed congress to stretch it's powers.
Current Examples:
Creation of National Bank, Military Draft, Air Force, building interstate highway system, prohibited racial discrimination etc.
The Elastic Clause Explained in 3 Minutes: The Constitution for Dummies Series