What makes thin stems?

I have a bunch of new crosses that are looking great. They are on their 4th set or so but the stem is thinner than it needs to be to hold itself up. Anything I might want to avoid/incorporate?

An intermittent breeze is the most common way to strengthen it...them

You possibly don't have enough light and it's "stretched itself thin".

Yup.. it's a light issue. Best bet in Vegas.

Get one or two 6500k CFL within 2 inches of them. Maybe tie her over and make her reach back up to the light until you get it grooving. The 6500k cfls really keep a plant short with tight internodes and veggie..

Light vs heat......summer problem......brutal here.

sometimes they just grow crazy fast when there a seedling. Remember that's the name of the game for a plant "get above all the other competing plants".
or as mentioned not enough light, or your light is to far away.

When you transplant. bury your plant up to the first true set of leaves.

Along with the reasons already mentioned in the other posts, sometimes its just genetics! For example, I just finished up a kandy kush and a utopia haze and while the Utopia haze was able to hold itself up perfectly, just about ever branch on the kandy kush had to be supported which is a common problem with the strain.

I gently squeeze my stems after the second set of leaves, crushing the fibers gently top to bottom,
it causes the stem to stiffen up very nicely.

sometimes they will need support after crushing the stem I use a 6" zip tie as a splint and some tephlon tape.

push zip tie into soil till end is below the top of the plant, wrap gently at 1 point just below a set of leaves.

Works great in conjunction with a nice breeze from a fan.

Keep the light as close as possible that is a major cause for stretch and weak stems.

In soil? Just bury more stem and crunch what you leave above the new soil.

Bless
DZ