system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do trees get water to their top portions without any sort of pump (like a heart)
gpt
Vertical plants generally have vascular (transport) tissues called xylem which carry water up from the roots to the leaves. Wood is one of these transport tissues. And as everybody mentioned, these work through capillary action, which sucks the water up like a straw. This is due in part to water itself. Because of something called hydrogen bonding, water molecules are actually really attracted to each other, and they want to stick together. If one water molecule evaporates through a leaf, the next one says "No, don't go!" and follows after it. This causes the next one to reach out, and the next, and the next, all the way down to the roots in a process called transpiration.  
**TL;DR: a one-way road to the top, and water molecules having the world's largest open relationship with themselves.**