Diameter
You loiter around a tombstone to measure its diameter. You’re doing this to each tombstone, and if you loiter around long enough, you’ll soon know the diameter of every tombstone.
Imagine loitering around a tombstone. You're leaning on it. You're staring at nothing. Oh, and you're also measuring its diameter.
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You need to know the diameter of each tombstone so you can make cakes (けい) for each one. But the cakes can't be wider than the tombstones, or they'll flop off onto the ground.
These carefully measured tombstone-diameter cakes are your special way of saying thanks to the ghosts for letting you loiter around their graves.