trigger ActivityHistoryToChatter on Task (after insert, after update) {
for(Task t : trigger.new)
{
    //if it's now a completed task, and wasn't  before
    //create the new chatter post.
    string what = t.whatID;

    if(what.substring(0,3) == 'a19')
    /*
        The above line lets you control exactly which objects you want to toss this on. Adjust the If statement to the
        3 digit object code for various elements.

        Ex:

        Cases - 500
        Accounts - 001
        Opportunities - 006

        I STRONGLY suggest you use this if you're going to do a larger, longer demo, as you'll need to chatter enable any
        object you play with the task history on or this will crash. If you're not playing with Activity History/Tasks anywhere
        else, won't be a big deal.
    */

    //{
        if(!t.alreadyPosted__c && t.status == 'Completed')//hasn't been posted, completed
        {
            Task daTask = [select id, description, whoid, activityDate, ownerid, subject from Task where id = :t.id];
            string whoelement = daTask.whoId;
            if (whoelement != NULL)
            {
            if(whoElement.substring(0,3) == '003')//contact
                {Contact who = [select name from Contact where id = :whoelement];
                whoElement = who.name;}
            else
                {Lead who2 = [select name from Lead where id = :whoelement];
                whoElement= who2.name;}
            }
        User owner = [select name from User where id = :daTask.OwnerId];

        daTask.alreadyPosted__c = true;

        FeedPost OppFeed = new FeedPost();
        string body = t.Subject + ' was completed by ' + owner.name + ' on ';
        body += t.activityDate.Month() + '/' + t.activityDate.Day() + '/' + t.ActivityDate.Year() + 'n';
        if(t.description != null)
            body += 'Comments: ' + t.description;
        OppFeed.body = body;
        oppFeed.parentID = t.whatID;
        oppFeed.linkURL = '/' + t.id;
        oppFeed.title = t.subject;
        insert oppFeed;
        }
    //}
}