Final answer:
The colonial militia in Lexington knew that the British were coming due to an alarm system and warnings from riders like Paul Revere. The Lexington militia and volunteers from the surrounding countryside were able to assemble and prepare for the British forces. The British troops were surprised to find that everyone on the road to Concord already knew they were coming.
Step-by-step explanation:
The colonial militia in Lexington knew that the British were coming because of a system of alarm that was put in place to alert the people of Massachusetts to the approach of the possibly hostile British forces. Paul Revere, William Dawes, and other riders spread the news, and bonfires, bells, and other means were used to signal the warning. As a result, the Lexington militia assembled, and more volunteers from the surrounding countryside joined them.
The British troops had hoped to catch the militia unaware, but they were surprised to find that everyone on the road to Concord already knew they were coming. This was because the local militia had received warnings and information from the riders and the alarm system that had been set up.
Colonel Smith, the British commander, had sent Major Pitcairn and his troops ahead to try and catch the militia by surprise, but their speed was not enough to outpace the informed and prepared colonial militia.