I use an analogy with USD. UTXOs are like the bills that I carry around in my wallet. If I go to buy a coffee and it costs $3, I can pay with $1, $5, $10, $20, or $100. I could choose any of them to pay for the coffee.

If I choose the $1 UTXO, then I would need 3 of them to pay for the coffee and I'd get no change back.

If I choose the $5 UTXO, then I get $2 change back (now itself a $2 UTXO for me to use in the future).

I could pay with the $100 too and get a $97 UTXO back, but now the vendor would know that I have at least $97 more on me. Similarly if you buy something with bitcoin that is a small amount of sats and use a UTXO worth much more, it works but you are revealing to the vendor that you have more bitcoin.

If they understand this, then I also add in this:

If there is something that you are buying that costs $50, it takes more time and energy to count out 50 $1 bills. It is much easier to just use a combination of larger bills or a single $50. In bitcoin terms, combining multiple smaller UTXOs is similar in that the more UTXOs you use, the larger the transaction becomes and the more expensive it is to send.