Not only are Duplo blocks *like* Lego blocks, they *are* Lego blocks, and I don't just mean that Duplo is a Lego brand and produced in the same factory, the two systems are actually compatible.

A Lego 2x2 block fits and sticks to the underside of a Duplo block, and the nob on the top of a Duplo block fits with the pillar in the middle under a Lego 2x2 block.

That's the reason the Duplo nobs are hollowed-out circles rather than the filled-in circles Lego blocks have! And it's why Duplo undersides have little tabs along the inner edge -- they're not necessary for fitting with Duplo nobs, but they're essential for grabbing Lego nobs!