Matt and Johnny both purchase pop and chips at the same convenience store. They have different tastes for pop and chips, so Matt buys 3 cans of pop and 10 bags of chips whereas Johnny buys 15 cans of pop and 2 bags of chips. If they both have well-behaved (monotonic and diminishing MRS) preferences, then Matt's marginal rate of substitution between cans of pop and bags of chips is lower than Johnny's.