You can either buy a pulled-out bed or a pop-up bed. The former, as the name suggests, is a low-lying bed that simply needs to be pulled out from under the main bed, whereas in case of the latter, the lower bed can be raised to the same height as that of the main bed. This way, you can turn the single bed into a double bed. While adults can sleep on the pop-up type, small trundle beds that can be pulled out work well for children.
Trundle beds are great space savers and truly a blessing when you have guests coming over for sleepovers, just make sure that you buy a comfortable mattress that will provide much-needed support to the spine.