The Bengals stayed home for a Thursday Night duel against the Texans.  In the first quarter the Texans jumped out to an early lead when Ka'imi Fairbairn kicked a 26-yard field goal to make it 3-0.  The Bengals tied it up in the second quarter when Randy Bullock kicked a 39-yard field goal to make it 3-3.  Though the Texans would then retake the lead when DeShaun Watson ran for a 49-yard touchdown to make it 10-3.  Bullock closed out the first half scoring with a 29-yard field goal to make it 10-6 at halftime.  In the third quarter, the Bengals went back to work when Bullock kicked a 30-yard field goal to get his team within a point making the score 10-9.  In the fourth quarter, the Texans moved ahead by 4 after Fairbairn kicked a 42-yard field goal to make it 13-9.  The Bengals tried to come back with seconds left but after a failed fourth down conversion attempt, the Texans were able to seal the victory. With the loss, the Bengals dropped to 0-2.  This would be the team's first 0-2 start since 2008. They are also the first team to open a season with two home games without scoring a touchdown since the 1939 Eagles.
Question: How many points were the Texans in the lead by during the second half?
Answer: 4