Replace the question mark with a number

Bill and Marla decided that the only way to pull off a Sunday afternoon quickie with their ten-year-old son in the apartment was to send him out on the balcony and tell him to report on all the neighborhood activities.

The boy began his commentary as his parents put their plan into operation. "There's a car being towed from the parking lot," he said. "An ambulance just drove by." A few moments passed.

"Looks like the Andersons have company," he called out. "Matt's riding a new bike, and the Coopers are having sex."

Mom and Dad shot up in bed. "How do you know that?" the startled father asked.

"Their kid is standing out on the balcony too," his son replied.