And here's Daddy tickling both Erin and Garrett.
When Garrett thinks he's going to be tickled or gets excited in any way, he draws his arms up to his neck and shrugs his shoulders. He kind of contracts in, very much like a young baby does when he stretches or gets tickled. This is another one of Garrett's quirks (like gagging himself) that nobody can explain.
A lesser version of the full arm/shoulder contraction is the fingers behind the ears contraction. When Garrett is just a little excited he will put his index fingers behind his ear lobes and push into his neck. Again, some kind of pleasurable sensation that only he can explain.