Bills become laws through a formal approval process that varies by country: In the United States:

• After a bill passes both the House of Representatives and the Senate, it is sent to the President.
• The President can sign the bill into law, veto it, or take no action.
• If the President signs the bill, it becomes law.
• If the President vetoes it, Congress can override the veto with a two-thirds majority in both chambers for the bill to become law without the President’s signature.
• If the President takes no action for 10 days while Congress is in session, the bill automatically becomes law; if Congress adjourns during that period, the bill does not become law ("pocket veto").
  Thus, in the U.S., the President signs bills to become laws

In the United Kingdom:

• After a bill passes both the House of Commons and the House of Lords, it must receive Royal Assent.
• Royal Assent is the formal approval by the monarch, which makes the bill an Act of Parliament (law).
• Although the monarch theoretically could refuse assent, this has not happened since 1708.
  Therefore, in the UK, the monarch signs bills to become laws

In Canada:

• Bills passed in identical form by the Senate and the House of Commons are presented to the Governor General.
• The Governor General grants Royal Assent on behalf of the Monarch, making the bill law.
• Royal Assent is typically given by the Governor General signing the bill or by a formal ceremony.
  Hence, in Canada, the Governor General signs bills to become laws on behalf of the Monarch

Summary:

Country| Who Signs Bills to Become Law
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United States| The President
United Kingdom| The Monarch (Royal Assent)
Canada| The Governor General (Royal Assent on behalf of the Monarch)

This formal signing or assent is the final step for a bill to become law in these systems.