SELECT Id,(SELECT Id FROM Opportunities) FROM Account. This SOQL works but not SELECT Id,(SELECT Id FROM Opportunity) FROM Account. Can anyone explain? Also, SELECT Id FROM Opportunity worked.

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Unable to see Launch option in hands on challenge in trailhead Install Apps and Packages in Your Trailhead Playground

Hello guys,

I am trying to do one of the examples in the Apex triggers where a SOQL query is used and I dont understand one of the parameters that is in line 6 which is:

06        [SELECT Id,(SELECT Id FROM Opportunities) FROM Account WHERE Id IN :Trigger.New]);

I dont understand the parameter "IN : Trigger.New])" ??, 

Complete Example:
01trigger AddRelatedRecord on Account(after insert, after update) {
02    List<Opportunity> oppList = new List<Opportunity>();
03     
04    // Get the related opportunities for the accounts in this trigger
05    Map<Id,Account> acctsWithOpps = new Map<Id,Account>(
06        [SELECT Id,(SELECT Id FROM Opportunities) FROM Account WHERE Id IN :Trigger.New]);
07     
08    // Add an opportunity for each account if it doesn't already have one.
09    // Iterate through each account.
10    for(Account a : Trigger.New) {
11        System.debug('acctsWithOpps.get(a.Id).Opportunities.size()=' + acctsWithOpps.get(a.Id).Opportunities.size());
12        // Check if the account already has a related opportunity.
13        if (acctsWithOpps.get(a.Id).Opportunities.size() == 0) {
14            // If it doesn't, add a default opportunity
15            oppList.add(new Opportunity(Name=a.Name + ' Opportunity',
16                                       StageName='Prospecting',
17                                       CloseDate=System.today().addMonths(1),
18                                       AccountId=a.Id));
19        }          
20    }
21 
22    if (oppList.size() > 0) {
23        insert oppList;
24    }
25}



I would really appreciate if anybody can help me. Thanks.