One of my favourite types of sex toy on the market today is the dual-density dildo, and the simple reason for this is that they offer an unparalleled sense of realism, something which you'll almost struggle to believe until the first time you try one for yourself.
What makes dual-density dildos so special is that they're designed in such a way that they have a firm-yet-flexible inner core, surrounded by a layer of soft squishy material on the outside. It works in much the same way as a real penis, where the shaft is solid during an erection, while the head remains spongy. It's not something which many of us ever give much thought to, but this particular feature of the penis exists so as to facilitate comfortable intercourse for the receptive partner; the spongy head acts like a shock absorber, cushioning any accidental impact caused by over-vigorous thrusting, and the dual-density dildos mimic this perfectly.