He ties it too loose

(chapter originally by SpiralFelix at writing.com)

Isabel continued to blush as she felt Steve's fingers moving around behind her neck.

"Thanks, Steve." Isabel said once he was done. She felt her breasts jiggle slightly more than before as she stood up again. Idly adjusting the top, Isabel noticed that all three of the guys were intently staring at her. More specifically, at her chest. Her blush intensifying, Isabel loudly proclaimed, "Well I could sure use a dip after that!" and nervously laughed as she began walking towards the waves.

"That sounds good to me!" said Dylan, snapping out of his stupor and quickly following after Isabel. The other two followed behind, keeping an eye on her ass as she walked ahead of them.

The four of them entered the water and began splashing and swimming. Once or twice, Isabel's nipple managed to peek out from her loosened bikini top, much to the surprise and delight of the boys. Isabel would quickly adjust the top and obscure her nipples from view again, but she felt more and more embarrassed each time. Soon, even a few strangers in the water were keeping their eyes on the blonde's top while she played. And they were treated to several more glimpses of Isabel's round breasts before she could cover them again.

Unbeknownst to Isabel, each time she shoved her breasts back under the cover of her top, she pulled it just a little bit looser. What had initially been a few inches of slack beyond the tie had been reduced to the knots at the end of the strings. The extra looseness meant Isabel was now exposing herself every few seconds.

Finally, she relented and said "Hang on, I've gotta retie this stupid thing." She turned away from the group and reached behind her neck, taking a string in each hand. Isabel quickly undid the barely holding together knot and began to tie it again, this time more securely.

But suddenly, Isabel was surprised by...