Mae and Angus Tuck were accustomed to change, nevertheless these latest changes upset them. Why?

Angus and Mae Tuck are accustomed to change happening around them, because they have each lived for more than a century.  They are immortal, which means that they do not change, but they see large changes in the world around them.  Additionally, because of their immortality, Angus and Mae Tuck have to move to a new home about every twenty years.  Otherwise people get suspicious.  

"But they can't stay on in any one place...

Angus and Mae Tuck are accustomed to change happening around them, because they have each lived for more than a century.  They are immortal, which means that they do not change, but they see large changes in the world around them.  Additionally, because of their immortality, Angus and Mae Tuck have to move to a new home about every twenty years.  Otherwise people get suspicious.  

"But they can't stay on in any one place for long, you know. None of us can. People get to wondering." She sighed. "We been in this house about as long as we dare, going on twenty years."

Change doesn't typically bother the Tucks, because the changes exist around them.  This time though, the changes directly impact the Tuck family and their secret. Winnie discovers Jesse drinking from the spring, and she wants to drink too.  He fumbles for an explanation as to why she shouldn't drink, but he can't come up with anything.  Luckily, Mae and Miles show up.  Together the three Tucks "kidnap" Winnie.  The Tuck family is upset for two reasons.  Number one, their spring has been discovered.  Number two, it has been discovered by a child, which makes things more difficult.  

"This is awful!" said Jesse. "Can't you do something, Ma? The poor little tad."

"We ought to've had some better plan than this," said Miles.

"That's the truth," said Mae helplessly. "The dear Lord knows there's been time enough to think of one, and it had to happen sooner or later. We been plain bone lucky it hasn't before now. But I never expected it'd be a child!"

The discovery of their spring and their secret is only compounded when the man in the yellow suit announces his intentions to market and sell the spring water to the highest bidder.  The Tucks, Angus especially, know the dangers of this possibility.