Final answer:
A double-ended wrench or spanner has a fixed opening at each end, making it versatile for fitting multiple sizes of nuts or bolts, similar to fitting specific shaped pegs through corresponding holes in a child's toy.
Step-by-step explanation:
A tool that has a fixed opening at each end that allows it to fit more than one size of nut or bolt is a double-ended wrench or sometimes called a spanner. This type of wrench is very versatile as it can be used on different sizes of fasteners. The design is comparable to the concept of a square peg fitting through the square hole and a round peg fitting through the round hole of a children's toy, where the pegs and holes are designed to correspond with each other in shape and size for a proper fit. In the case of the wrench, each end is designed to fit nuts or bolts of specific sizes, so one end might fit a larger size while the other fits a smaller size. These wrenches are commonly used in various applications, from household repairs to automotive maintenance.