When they had (the original) order of the Phoenix meetings, they had this game of who had to watch Harry. Before you headed in, you had to try and not get "tagged" by whoever had Harry. Sirius was the last person before the order of the Phoenix was closed.
When Sirius was pulled by the veil, he screamed. Most people thought it was just a sort of aaaahhhhhh scream. No. He yelled what only Remus would understand. Tag, your it. This was why Remus risked his life for Harry SO much. He was it. He had to take care of Harry.

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PJO and HoO head cannons
FanfictionWeird things of PJO and HoO There may be a throw in of other fandoms once in a while BTDubs. Near the 100's it gets a bit more Harry Potter