1 Ginny: Erwin's right, don't shoot them. Leave them here.
2 Ginny: By breaking into the Library, they've triggered da Vinci's final and most diabolical deathtrap, which will finish them horribly within the hour.
3 Monty: But... traps always come in threes! There was the door trapped with the giant frog, the rake, and the enormous stone ball!
4 Ginny: You thought the rake was a trap?! Hah ha ha!
4 Prof. Jones: Oh! So that explains the lawnmower.

Now it all makes sense!

I get an unusual amount of pleasure in wrapping a piece of string around Lego characters to make it look like they're tied up with rope. There's actually quite an art to it.

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2018-07-12 Rerun commentary: The first time around, several people didn't get the joke in this strip. The idea is that the rake wasn't intended to be one of Leonardo's traps. After all, who would use a rake as a deliberate trap? It's only a minor gardening implement, after all.

But a lawnmower is a much more serious piece of gardening equipment, so one that you could indeed set up as a deliberate trap. And the implication is that they indeed ran across a lawnmower at some point, which we just didn't get to see, and that they were mystified by it at the time.