Why Wait?

We've been studying the book of Acts & the early church in our teen Sunday school class. (We haven't gotten very far at this point. We've been in the first chapter for over a month.) In Acts 1:4, Jesus told the disciples to go back to Jerusalem and "wait for the promise of the Father." 10 days later "when the day of Pentecost was fully come," they finally experienced the promise of the Father. An interesting question, that I don't have the answer for is, why did Jesus want them to wait for 10 days? Why didn't they recieve the power of the Holy Spirit right there on Mt. Olives as Jesus ascended back into Heaven? Think about that one today.