There are three branches of government. Why?

Why this is the answer

During the Constitutional Convention, the Founders feared that any person or group holding too much power could take over the country. Their answer was to divide the work of government. Congress makes federal laws, the executive branch led by the President enforces them, and the courts review them. That division is called separation of powers. Dividing power was only half of the design. Each branch was also given ways to block, or threaten to block, the actions of the others, a system known as checks and balances. The Constitution is full of examples. The President can veto a bill, and Congress can still pass it if two-thirds of each house vote again in favor. The President makes treaties, but two-thirds of the senators present must agree. The President nominates Supreme Court justices and other top officials, and they take office only with the consent of the Senate. The Supreme Court, in turn, can reject a law passed by Congress. Federalist No. 51, one of the essays published in 1788 to win support for the Constitution, explains the logic with a famous line: ambition must be made to counteract ambition. The same essay observes that if men were angels, no government would be necessary. The Constitution does not count on officials being saints. It sets each branch's interests against the others so that none can dominate, and in doing so it limits government and protects the rights of the people. The three official answers describe the same idea from different angles: so one part does not become too powerful, checks and balances, and separation of powers. Any of them is correct. "Checks and balances" is short and easy to say clearly. The tempting wrong answers sound practical, for example that three branches make laws faster or give each political party its own branch. Neither matches the reason the Founders gave, which was to keep any one person or group from gaining too much power. Picture three people who each hold one key to a shared safe: none can open it alone, and none can lock the others out.