Graduate
A person puts a lid on their cross after they graduate. It’s a symbolic gesture, putting the lid over the cross, but it’s just what you do when you graduate around these parts.
Imagine a wooden cross and picture yourself putting a lid over it. Visualize this motion in your mind and imagine having to do something like this at your graduation.
そつ
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Oops, looks like you forgot your lid. Without the lid, you’ll be held back. So two (そつ) more years now until you can graduate. So two more years, eh? Just for forgetting a lid? Nonsense!
Imagine being held back two more years from graduation. You’d lose all your motivation. Imagine how lousy this would be.